A leap year (or intercalary year) is a year containing one or more extra days (or, in the case of lunisolar calendars, an extra month) in order to keep the calendar year synchronised with the astronomical or seasonal year. For example, in the Gregorian calendar, February would have 29 days in a leap year instead of the usual 28 - and consequently, the whole year would last 366 days instead of the common 365. Because seasons and astronomical events do not repeat at an exact number of full days, a calendar which had the same number of days in each year would, over time, drift with respect to the event it was supposed to track. By occasionally inserting (or intercalating) an additional day or month into the year, the drift can be corrected. A year which is not a leap year is called a common year. Roscoe Conkling Arbuckle (March 24, 1887 – June 29, 1933) was an American silent film comedian. ... Leap Year is a 1921 comedy film directed by and starring Fatty Arbuckle. ... A lunisolar calendar is a calendar whose date indicates both the moon phase and the time of the solar year. ... According to the Gregorian calendar, the calendar year begins on January 1 and ends on December 31. ... A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ... The seasonal year is the time between successive recurrences of a seasonal event such as the flooding of a river, the migration of a species of bird, or the flowering of a species of plant. ... For the calendar of religious holidays and periods, see liturgical year. ... Intercalation is the insertioffn of an extra day, week or month into some calendar years to make the calendar follow the seasons. ... A common year is a year that is common calendar year. ...

Gregorian calendar

In the Gregorian calendar, the current standard calendar in most of the world, most years whose division by 4 equals an integer are leap years. In one leap year, the month of February has 29 days instead of 28. Adding an extra day to the calendar every four years compensates for the fact that a solar year is almost 6 hours longer than 365 days. For the calendar of religious holidays and periods, see liturgical year. ... Solar year The period of time required for the earth to make one complete revolution around the sun, measured from one vernal equinox to the next. ...

However, some exceptions to this rule are required since the duration of a solar year is slightly less than 365.25 days. Years which are evenly divisible by 100 are not leap years, unless they are also evenly divisible by 400, in which case they are leap years.^[1][2] For example, 1600 and 2000 were leap years, but 1700, 1800 and 1900 were not. Similarly, 2100, 2200, 2300, 2500, 2600, 2700, 2900, and 3000 will not be leap years, but 2400 and 2800 will be. By this rule, the average number of days per year will be 365 + 1/4 − 1/100 + 1/400 = 365.2425, which is 365 days, 5 hours, 49 minutes, and 12 seconds.

The Gregorian calendar was designed to keep the vernal equinox on or close to March 21, so that the date of Easter (celebrated on the Sunday after the 14th day of the Moon that falls on or after 21 March) remains correct with respect to the vernal equinox.^[3] The vernal equinox year is about 365.242374 days long (and increasing), whereas the average year length of the Gregorian calendar is 365.2425. Illumination of Earth by Sun on the day of equinox The vernal equinox (or spring equinox) marks the beginning of astronomical spring. ... is the 80th day of the year (81st in leap years) in the Gregorian calendar. ... This article is about the Christian festival. ... is the 80th day of the year (81st in leap years) in the Gregorian calendar. ...

The marginal difference of 0.000125 days means that in around 8,000 years, the calendar will be about one day behind where it is now. But in 8,000 years, the length of the vernal equinox year will have changed by an amount which cannot be accurately predicted (see below). Therefore, the current Gregorian calendar suffices for practical purposes, and Herschel's correction (making 4000 AD not a leap year) will probably not be necessary. John Herschel Sir John Frederick William Herschel (7 March 1792 – 11 May 1871) was an English mathematician and astronomer. ...

This graph shows the variation between the seasonal year versus the calendar year due to unequally spaced 'leap days' rules. See Iranian calendar to contrast with a calendar based on 8 leap days every 33 years.

Seasonal error of Gregorian calendar Originally created by Tom Ruen, 2003, using MSExcel graph, labeled with MSPaint Converted from Image:Gregoriancalendarleap. ... The Iranian calendar (Persian: ), also known as Persian calendar or (mistakenly) the Jalāli Calendar is an astronomical solar calendar currently used in Iran and Afghanistan as the main official calendar. ...

Algorithm

This algorithm determines leap years on the proleptic Gregorian calendar, which includes leap years before the official inception in 1582. The proleptic Gregorian calendar is produced by extending the Gregorian Calendar to dates preceding its official introduction in 1582. ... Gregorian Calendar switch: Year 1582 involved conversion to the Gregorian calendar. ...

Pseudocode to determine whether a year is a leap year or not: Pseudocode (derived from pseudo and code) is a compact and informal high-level description of a computer programming algorithm that uses the structural conventions of programming languages, but omits detailed subroutines, variable declarations or language-specific syntax. ...

 if year modulo 400 is 0 then leap else if year modulo 100 is 0 then no_leap else if year modulo 4 is 0 then leap else no_leap 

A more direct algorithm (terms may be grouped either way): In computing, the modulo operation finds the remainder of division of one number by another. ...

 if (year modulo 4 is 0) and (year modulo 100 is not 0) or (year modulo 400 is 0) then leap else no_leap 

Leap day

Main article: February 29

February 29 is a date that usually occurs every four years, and is called leap day. This day is added to the calendar in leap years as a corrective measure, because the earth does not orbit around the sun in precisely 365 days. February 29 is a day added into a leap year of the Gregorian calendar. ...

The Gregorian calendar is a modification of the Julian calendar first used by the Romans. The Roman calendar originated as a lunisolar calendar and named many of its days after the syzygies of the moon: the new moon (Kalendae or calends, hence "calendar") and the full moon (Idus or ides). The Nonae or nones was not the first quarter moon but was exactly one nundinae or Roman market week of nine days before the ides, inclusively counting the ides as the first of those nine days. In 1825, Ideler believed that the lunisolar calendar was abandoned about 450 BC by the decemvirs, who implemented the Roman Republican calendar, used until 46 BC. The days of these calendars were counted down (inclusively) to the next named day, so 24 February was ante diem sextum Kalendas Martii ("the sixth day before the calends of March") often abbreviated a. d. VI Kal. Mar. The Romans counted days inclusively in their calendars, so this was actually the fifth day before March 1 when counted in the modern exclusive manner (not including the starting day).^[4] The Julian calendar was a reform of the Roman calendar which was introduced by Julius Caesar in 46 BC and came into force in 45 BC (709 ab urbe condita). ... The Roman calendar changed its form several times in the time between the foundation of Rome and the fall of the Roman Empire. ... A lunisolar calendar is a calendar whose date indicates both the moon phase and the time of the solar year. ... Look up Syzygy in Wiktionary, the free dictionary. ... For other uses, see Full Moon. ... Lunar phase refers to the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. ... Christian Ludwig Ideler (September 21, 1766 - August 10, 1846), German chronologist and astronomer, was born near Perleberg on the 21st of September 1766. ... Centuries: 6th century BC - 5th century BC - 4th century BC Decades: 500s BC 490s BC 480s BC 470s BC 460s BC - 450s BC - 440s BC 430s BC 420s BC 410s BC 400s BC Years: 455 BC 454 BC 453 BC 452 BC 451 BC - 450 BC - 449 BC 448 BC... Decemviri (sing. ... The Roman calendar changed its form several times in the time between the foundation of Rome and the fall of the Roman Empire. ... is the 55th day of the year in the Gregorian calendar. ... Ancient Rome was a civilization that grew from a small agricultural community founded on the Italian Peninsula circa the 9th century BC to a massive empire straddling the Mediterranean Sea. ... is the 60th day of the year (61st in leap years) in the Gregorian calendar. ...

The Republican calendar's intercalary month was inserted on the first or second day after the Terminalia (a. d. VII Kal. Mar., February 23). The remaining days of Februarius were dropped. This intercalary month, named Intercalaris or Mercedonius, contained 27 days. The religious festivals that were normally celebrated in the last five days of February were moved to the last five days of Intercalaris. Because only 22 or 23 days were effectively added, not a full lunation, the calends and ides of the Roman Republican calendar were no longer associated with the new moon and full moon. In Roman mythology, Terminus was the god of boundaries. ... is the 54th day of the year in the Gregorian calendar. ... Mercedonius was a month in the ancient Roman calendar. ...

The Julian calendar, which was developed in 46 BC by Julius Caesar, and became effective in 45 BC, distributed an extra ten days among the months of the Roman Republican calendar. Caesar also replaced the intercalary month by a single intercalary day, located where the intercalary month used to be. To create the intercalary day, the existing ante diem sextum Kalendas Martii (February 24) was doubled, producing ante diem bis sextum Kalendas Martii. Hence, the year containing the doubled day was a bissextile (bis sextum, "twice sixth") year. For legal purposes, the two days of the bis sextum were considered to be a single day, with the second half being intercalated, but common practice by 238, when Censorinus wrote, was that the intercalary day was followed by the last five days of February, a. d. VI, V, IV, III and pridie Kal. Mar. (which would be those days numbered 24, 25, 26, 27, and 28 from the beginning of February in a common year), i.e. the intercalated day was the first half of the doubled day. All later writers, including Macrobius about 430, Bede in 725, and other medieval computists (calculators of Easter), continued to state that the bissextum (bissextile day) occurred before the last five days of February. The Julian calendar was a reform of the Roman calendar which was introduced by Julius Caesar in 46 BC and came into force in 45 BC (709 ab urbe condita). ... Centuries: 2nd century BC - 1st century BC - 1st century Decades: 90s BC 80s BC 70s BC 60s BC 50s BC - 40s BC - 30s BC 20s BC 10s BC 0s BC 0s Years: 51 BC 50 BC 49 BC 48 BC 47 BC 46 BC 45 BC 44 BC 43 BC... For other uses, see Julius Caesar (disambiguation). ... Centuries: 2nd century BC - 1st century BC - 1st century Decades: 90s BC 80s BC 70s BC 60s BC 50s BC - 40s BC - 30s BC 20s BC 10s BC 0s BC 0s Years: 50 BC 49 BC 48 BC 47 BC 46 BC 45 BC 44 BC 43 BC 42 BC... is the 55th day of the year in the Gregorian calendar. ... Censorinus, Roman grammarian and miscellaneous writer, flourished during the 3rd century AD. He was the author of a lost work De Accentibus and of an extant treatise De Die Natali, written in 238, and dedicated to his patron Quintus Caerellius as a birthday gift. ... Ambrosius Theodosius Macrobius, Roman grammarian and philosopher, flourished during the reigns of Honorius and Arcadius (395-423). ... For other uses, see Bede (disambiguation). ... Computus (Latin for computation) is the calculation of the date of Easter in the Christian calendar. ... This article is about the Christian festival. ...

Until 1970, the Roman Catholic Church always celebrated the feast of Saint Matthias on a. d. VI Kal. Mar., so if the days were numbered from the beginning of the month, it was named February 24 in common years, but the presence of the bissextum in a bissextile year immediately before a. d. VI Kal. Mar. shifted the latter day to February 25 in leap years, with the Vigil of St. Matthias shifting from February 23 to the leap day of February 24. Other feasts normally falling on February 25–28 in common years are also shifted to the following day in a leap year (although they would be on the same day according to the Roman notation). The practice is still observed by those who use the older calendars. Catholic Church redirects here. ... This article is about the New Testament figure. ... is the 55th day of the year in the Gregorian calendar. ... is the 56th day of the year in the Gregorian calendar. ... Vigil, tacuinum sanitatis casanatensis (XIV century) This article is about the period of sleeplessness. ... is the 54th day of the year in the Gregorian calendar. ... is the 55th day of the year in the Gregorian calendar. ...

Julian, Coptic and Ethiopian calendars

The Julian calendar adds an extra day to February in years evenly divisible by four. The Julian calendar was a reform of the Roman calendar which was introduced by Julius Caesar in 46 BC and came into force in 45 BC (709 ab urbe condita). ...

The Coptic calendar and Ethiopian calendar also add an extra day to the end of the year once every four years before a Julian 29-day February. The Coptic calendar, also called the Alexandrian calendar, is used by the Coptic Orthodox Church. ... The Ethiopian calendar (Amharic: የኢትዮጵያ ዘመን አቆጣጠር ), also called the Geez calendar, is the principal calendar used in Ethiopia and is also the liturgical year of Christians in Eritrea belonging to the Eritrean Orthodox Tewahdo Church, Eastern Catholic Church of Eritrea and Lutheran (Evangelical Church of Eritrea), where it is commonly known...

This rule gives an average year length of 365.25 days. However, it is 11 minutes longer than a real year. This means that the vernal equinox moves a day earlier in the calendar every 131 years.

Revised Julian calendar

The Revised Julian calendar adds an extra day to February in years divisible by four, except for years divisible by 100 that do not leave a remainder of 200 or 600 when divided by 900. This rule agrees with the rule for the Gregorian calendar until 2799. The first year that dates in the Revised Julian calendar will not agree with those in the Gregorian calendar will be 2800, because it will be a leap year in the Gregorian calendar but not in the Revised Julian calendar. The Revised Julian calendar is a calendar that was considered for adoption by the Eastern Orthodox churches at a synod in Istanbul in May 1923. ...

This rule gives an average year length of 365.242222… days. This is a very good approximation to the mean tropical year, but because the vernal equinox year is slightly longer, the Revised Julian calendar does not do as good a job as the Gregorian calendar of keeping the vernal equinox on or close to 21 March. A tropical year is the length of time that the Sun, as viewed from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere). ... is the 80th day of the year (81st in leap years) in the Gregorian calendar. ...

Chinese calendar

The Chinese calendar is lunisolar, so a leap year has an extra month, often called an embolismic month after the Greek word for it. In the Chinese calendar the leap month is added according to a complicated rule, which ensures that month 11 is always the month that contains the northern winter solstice. The intercalary month takes the same number as the preceding month; for example, if it follows the second month (二月) then it is simply called "leap second month" (traditional Chinese: 閏二月; simplified Chinese: 闰二月; pinyin: rùn'èryuè). The Chinese calendar is a lunisolar calendar, incorporating elements of a lunar calendar with those of a solar calendar. ... A lunisolar calendar is a calendar whose date indicates both the moon phase and the time of the solar year. ... Intercalation is the insertion of an extra day or month into some calendar years to make the calendar follow the seasons. ... “Summer solstice” redirects here. ... Traditional Chinese characters refers to one of two standard sets of printed Chinese characters. ... Simplified Chinese character (Simplified Chinese: or ; traditional Chinese: or ; pinyin: or ) is one of two standard sets of Chinese characters of the contemporary Chinese written language. ... Pinyin, more formally called Hanyu Pinyin (Simplified Chinese: ; Traditional Chinese: ; Pinyin: ), is the most common variant of Standard Mandarin romanization system in use. ...

Hebrew calendar

The Hebrew calendar is also lunisolar with an embolismic month. This extra month is called Adar Alef (first Adar) and is added before Adar, which then becomes Adar Bet (second Adar). According to the Metonic cycle, this is done seven times every nineteen years (specifically, in years 3, 6, 8, 11, 14, 17, and 19). The Hebrew calendar (‎) or Jewish calendar is the calendar used by Jews for religious purposes. ... Adar (אֲדָר, Standard Hebrew Adar, Tiberian Hebrew ʾĂḏār: from Akkadian adaru) is the sixth month of the ecclesiastical year and the twelfth month of the civil year on the Hebrew calendar. ... Adar (אֲדָר, Standard Hebrew Adar, Tiberian Hebrew ʾĂḏār: from Akkadian adaru) is the sixth month of the religious year and the twelfth month of the civil year on the Hebrew calendar. ... Adar (אֲדָר, Standard Hebrew Adar, Tiberian Hebrew ʾĂḏār: from Akkadian adaru) is the sixth month of the ecclesiastical year and the twelfth month of the civil year on the Hebrew calendar. ... The Metonic cycle or Enneadecaeteris in astronomy and calendar studies is a particular approximate common multiple of the year (specifically, the seasonal tropical year) and the synodic month. ...

In addition, the Hebrew calendar has postponement rules that postpone the start of the year by one or two days. These postponement rules reduce the number of different combinations of year length and starting days of the week from 28 to 14, and regulate the location of certain religious holidays in relation to the Sabbath. In particular, the first day of the Hebrew year can never be Sunday, Wednesday or Friday. This rule is known in Hebrew as "lo adu rosh", i.e. "Rosh [ha-Shanah, first day of the year] is not Sunday, Wednesday or Friday" (as the Hebrew word adu is written by three Hebrew letters signifying Sunday, Wednesday and Friday). Accordingly, the first day of Pesah (Passover) is never Monday, Wednesday or Friday. This rule is known in Hebrew as "lo badu Pesah", which has a double meaning — "Pesah is not a legend", but also "Pesah is not Monday, Wednesday or Friday" (as the Hebrew word badu is written by three Hebrew letters signifying Monday, Wednesday and Friday). This article is about days of the week. ... For other uses, see Sabbath. ... Note: This article contains special characters. ... Passover, also known as Pesach or Pesah (פסח pesaḥ), is a Jewish holiday (lasting seven days in Israel and among some liberal Diaspora Jews, and eight days among other Diaspora Jews) that commemorates the exodus and freedom of the Israelites from Egypt; it is also observed by some Christians to...

One reason for this rule is that Yom Kippur, the holiest day in the Hebrew calendar, must never be adjacent to the weekly Sabbath (which is Saturday), i.e. it must never fall on Friday or Sunday, in order not to have two adjacent Sabbath days. However, Yom Kippur can be on Saturday. Yom Kippur (Hebrew:יוֹם כִּפּוּר ) is a Jewish holiday, known in English as the Day of Atonement. ... For other uses, see Sabbath. ...

Years consisting of 12 months have between 353 and 355 days. In a k'sidra ("in order") 354-day year, months have alternating 30 and 29 day lengths. In a chaser ("lacking") year, the month of Kislev is reduced to 29 days. In a malei ("filled") year, the month of Cheshvan is increased to 30 days. 13-month years follow the same pattern, with the addition of the 30-day Adar Alef, giving them between 383 and 385 days. Kislev (or Chisleu) (Hebrew: כִּסְלֵו, Standard Kislev Tiberian  ; from Akkadian kislimu) is the third month of the ecclesiastical year and the ninth month of the civil year on the Hebrew calendar. ... Cheshvan (חֶשְׁוָן, Standard Hebrew ḤeÅ¡van, Tiberian Hebrew ḪeÅ¡wān, ḤeÅ¡wān, short for מַרְחֶשְׁוָן, Standard Hebrew MarḥeÅ¡van, Tiberian Hebrew MarḫeÅ¡wān, MarḥeÅ¡wān: from Akkadian waraḫsamnu, literally eighth month) is the second month of the ecclesiastical year and the eighth month of...

Islamic calendar

In the Islamic calendar, leap months are not used. The Qur'an says: The Islamic calendar or Muslim calendar (Arabic: التقويم الهجري; at-taqwīm al-hijrī; Persian: تقویم هجري قمری ‎ taqwīm-e hejri-ye qamari; also called the Hijri calendar) is the calendar used to date events in many predominantly Muslim countries, and used by Muslims everywhere to determine the proper day on which to celebrate... The Qur’ān [1] (Arabic: , literally the recitation; also sometimes transliterated as Quran, Koran, or Al-Quran) is the central religious text of Islam. ...

Calendars with Leap Years synchronized with Gregorian

The Indian National Calendar and the Revised Bangla Calendar of Bangladesh organise their leap years so that the leap day is always close to February 29 in the Gregorian calendar. This makes it easy to convert dates to or from Gregorian. The Indian national calendar (sometimes called Saka calendar) is the official civil calendar in use in India. ... The Bangla calendar also known as Bônggabdo in the Bengali language, is the traditional calendar used in Bangladesh and Bangla-speaking regions of India. ... February 29 is a day added into a leap year of the Gregorian calendar. ... For the calendar of religious holidays and periods, see liturgical year. ...

The Bahá'í calendar is structured such that the leap day always falls within Ayyám-i-Há, a period of four or five days corresponding to Gregorian February 26 – March 1. Because of this, Baha'i dates consistently line up with exactly one Gregorian date. The Baháí calendar, also called the Badí‘ calendar, used by the Baháí Faith, is a solar calendar with regular years of 365 days, and leap years of 366 days. ... Ayyám-i-Há refers to a period of four or five intercalary days in the Baháí calendar, where Baháís celebrate the Festival of Ayyám-i-Há.[1] The four or five days occur in between the 18th and 19th months of the calender from February 26 to... is the 57th day of the year in the Gregorian calendar. ... is the 60th day of the year (61st in leap years) in the Gregorian calendar. ...

The Thai solar calendar uses the Buddhist Era (BE), but has been synchronized with the Gregorian since AD 1941. The Thai solar, or Suriyakati (สุริยคติ), calendar is used in traditional and official contexts in Thailand, although the Western calendar is sometimes used in business. ... The Thai solar, or Suriyakati (สุริยคติ), calendar is used in traditional and official contexts in Thailand, although the Western calendar is sometimes used in business. ... Look up AD, ad-, and ad in Wiktionary, the free dictionary. ... For other uses, see 1941 (disambiguation). ...

Hindu calendar

In the Hindu calendar, which is a lunisolar calendar, the embolismic month is called adhika maas (extra month). It is the month in which the sun is in the same sign of the stellar zodiac on two consecutive dark moons. Adhika Maas typically occurs once every 3 years, or 4 times over 11 years. So the yearly lag of a lunar year (which tends to have ten fewer days (355-356 days) per year than solar calendar) is adjusted every 3 years. Thus, Hindu festivals tend to occur within a given span. For example: the No Moon during Diwali festival tends to occur between October 22 and November 15. A page from the Hindu calendar 1871-72. ... A lunisolar calendar is a calendar whose date indicates both the moon phase and the time of the solar year. ...

Iranian calendar

The Iranian calendar also has a single intercalated day once in every four years, but every 33 years or so the leap years will be five years apart instead of four years apart. The system used is more accurate and more complicated, and is based on the time of the March equinox as observed from Tehran. The 33-year period is not completely regular; every so often the 33-year cycle will be broken by a cycle of 29 or 37 years. The Iranian calendar (Persian: ), also known as Persian calendar or (mistakenly) the Jalāli Calendar is an astronomical solar calendar currently used in Iran and Afghanistan as the main official calendar. ... For other uses, see Tehran (disambiguation). ...

Long term leap year rules

The accumulated difference between the Gregorian calendar and the vernal equinoctial year amounts to 1 day in about 8,000 years. This suggests that the calendar needs to be improved by another refinement to the leap year rule: perhaps by avoiding leap days in years evenly divisible by 8,000.

(The most common such proposal is to avoid leap years in years evenly divisible by 4,000.^[5] This is based on the difference between the Gregorian calendar and the mean tropical year. Others claim, erroneously, that the Gregorian calendar itself already contains a refinement of this kind.^[6])

A system of 128-year-based leap years has been proposed,^[7] and it can be adopted directly without any modification to current leap year calculations until the year 2048 because no year between now and 2048 is divisable by 128. This rule gives a mean year of 365 + 1/4 − 1/128 = 365.2578125 days, which is 365 days, 6 hours, 11 minutes, and 15 seconds.

However, there is little point in planning a calendar so far ahead because over a timescale of tens of thousands of years the number of days in a year will change for a number of reasons, most notably:

1. Precession of the equinoxes moves the position of the vernal equinox with respect to perihelion and so changes the length of the vernal equinoctial year.
2. Tidal acceleration from the sun and moon slows the rotation of the earth, making the day longer.

In particular, the second component of change depends on such things as post-glacial rebound and sea level rise due to climate change. We can't predict these changes accurately enough to be able to make a calendar that will be accurate to a day in tens of thousands of years. Precession of the equinoxes refers to the precession of the Earths axis of rotation. ... This article is about several astronomical terms (apogee & perigee, aphelion & perihelion, generic equivalents based on apsis, and related but rarer terms. ... It has been suggested that Tidal friction be merged into this article or section. ... Changes in the elevation of Lake Superior due to glaciation and post-glacial rebound Post-glacial rebound (sometimes called continental rebound, isostatic rebound or isostatic adjustment) is the rise of land masses that were depressed by the huge weight of ice sheets during the last ice age, through a process... Sea level measurements from 23 long tide gauge records in geologically stable environments show a rise of around 20 centimeters per century (2 mm/year). ... Variations in CO2, temperature and dust from the Vostok ice core over the last 450,000 years For current global climate change, see Global warming. ...

Folk traditions

In the English speaking world, it is a tradition that women may propose marriage only on leap years. While it has been argued that the tradition was initiated by Saint Patrick or Brigid of Kildare in 5th century Ireland, it is dubious as the tradition has not been attested before the 19th century.^[8] Supposedly, a 1288 law by Queen Margaret of Scotland (then age five and living in Norway), required that fines be levied if a marriage proposal was refused by the man; compensation ranged from a kiss to £1 to a silk gown, in order to soften the blow.^[9] Because men felt that put them at too great a risk, the tradition was in some places tightened to restricting female proposals to the modern leap day, 29 February, or to the medieval leap day, 24 February. According to Felten: "A play from the turn of the 17th century, 'The Maydes Metamorphosis,' has it that 'this is leape year/women wear breeches.' A few hundred years later, breeches wouldn't do at all: Women looking to take advantage of their opportunity to pitch woo were expected to wear a scarlet petticoat -- fair warning, if you will."^[10] For other uses, see Tradition (disambiguation). ... St Patrick redirects here, for other uses, see St. ... Saint Brigid redirects here. ... Europe in 450 The 5th century is the period from 401 to 500 in accordance with the Julian calendar in the Christian Era. ... Alternative meaning: Nineteenth Century (periodical) (18th century — 19th century — 20th century — more centuries) As a means of recording the passage of time, the 19th century was that century which lasted from 1801-1900 in the sense of the Gregorian calendar. ... Events February 22 - Nicholas IV becomes Pope. ... Margaret (9 April 1283– 26 September 1290), usually known as the Maid of Norway (Norwegian: , literally The Virgin of Norway), sometimes known as Margaret of Scotland (Margrete av Skottland), was a Norwegian–Scottish princess who is widely considered to have been Queen of Scots from 1286 until her death, although... February 29 is a day added into a leap year of the Gregorian calendar. ... is the 55th day of the year in the Gregorian calendar. ...

In Denmark, the tradition is that women may propose on leap day 24 February and that refusal must be compensated with 12 pairs of gloves. is the 55th day of the year in the Gregorian calendar. ...

In Greece, it is believed that getting married in a leap year is bad luck for the couple^{[citation needed]}. Thus, mainly in the middle of the past century, couples avoided setting a marriage date in a leap year. This article is about fortune. ...

1908 postcard

1908 postcard

Birthdays

A person born on February 29 may be called a "leapling" or a "leaper" ^[11]. In common years they usually celebrate their birthdays on 28 February or 1 March. February 29 is a day added into a leap year of the Gregorian calendar. ... For other uses, see Birthday (disambiguation). ... is the 59th day of the year in the Gregorian calendar. ... is the 60th day of the year (61st in leap years) in the Gregorian calendar. ...

For legal purposes, their legal birthdays depend on how different laws count time intervals. In Taiwan, for example, the legal birthday of a leapling is 28 February in common years, so a Taiwanese leapling born on February 29, 1980 would have legally reached 18 years old on February 28, 1998. is the 59th day of the year in the Gregorian calendar. ... February 29 is a day added into a leap year of the Gregorian calendar. ... Year 1980 (MCMLXXX) was a leap year starting on Tuesday (link displays the 1980 Gregorian calendar). ... is the 59th day of the year in the Gregorian calendar. ... Year 1998 (MCMXCVIII) was a common year starting on Thursday (link will display full 1998 Gregorian calendar). ...

In some situations, March 1 is used as the birthday in a non-leap year since it then is the day just after February 28.

There are many instances in children's literature where a person's claim to be only a quarter of their actual age turns out to be based on counting their leap-year birthdays. A similar device is used in the plot of Gilbert and Sullivan's 1879 comic opera The Pirates of Penzance: As a child, Frederic was apprenticed to a band of pirates until the age of 21. Now, having passed his 21st year, he leaves the pirate band and falls in love. However, it turns out that the pirate indenture says that his apprenticeship does not end until his 21st birthday, and since he was born on February 29, that day will not arrive until he is in his eighties, and so he must leave his fiancée and return to the pirates. Of course, it all turns out happily in the end. W. S. Gilbert Arthur Sullivan Gilbert and Sullivan refers to the Victorian era partnership of librettist W. S. Gilbert (1836–1911) and composer Arthur Sullivan (1842–1900). ... Comic opera, or light opera, denotes a sung dramatic work of a light or comic nature, usually with a happy ending. ... Drawing of the Act I finale The Pirates of Penzance, or The Slave of Duty, is a comic opera in two acts, with music by Arthur Sullivan and libretto by W. S. Gilbert. ... If youre looking for the TV show, see The Apprentice. ... Look up pirate and piracy in Wiktionary, the free dictionary. ... An Indentured servant is an unfree labourer under contract to work (for a specified amount of time) for another person, often without any pay, but in exchange for accommodation, food, other essentials and/or free passage to a new country. ...

According to DC Comics editor Julius Schwartz, the birth date of Superman is February 29th. This was chosen both as a way to keep the character young as well as account for the differences between Earth years and Kryptonian years. DC Comics is an American comic book and related media company. ... Julius Julie Schwartz (June 19, 1915 – February 8, 2004) was a comic book and pulp magazine editor, and a science fiction agent and prominent fan. ... Superman is a fictional character and comic book superhero , originally created by American writer Jerry Siegel and Canadian artist Joe Shuster and published by DC Comics. ...

See also

• Leap week calendar
• Leap second
• List of leap years
• Zeller's congruence

The leap week calendar is a reformed calendar system with a whole number of weeks every year, and with every year starting on the same weekday. ... A leap second is a one-second adjustment to civil time in order to keep it close to the mean solar time. ... Zellers congruence is an algorithm devised by Christian Zeller to calculate the day of the week for any calendar date. ...

References

For the Chinese civilization, see China. ...

External links

• Famous persons born on Leap Day

v • d • e   Time
Major concepts	Time · Eternity · Arguments for eternity · Immortality Deep time · History · Past · Present · Future · Futurology	Time Portal
Measurement and Standards	Chronometry · UTC · UT · TAI · Second · Minute · Hour · Sidereal time · Solar time · Time zone Clock · Horology · History of timekeeping devices · Astrarium · Marine chronometer · Sundial · Water clock This article is about the concept of time. ... This article is about the concept of time. ... While in the popular mind, eternity often simply means existing for an infinite, i. ... Arguments for eternity composed a particularly important area of philosophical debate among Greek, Jewish, Islamic, and Christian philosophers during the ancient and medieval periods. ... The Fountain of Eternal Life in Cleveland, Ohio Immortality (or eternal life) is the concept of living in physical or spiritual form for an infinite length of time, or in a state of timelessness. ... Deep time is the theory that Earth is billions of years old and thus had a long history of development and change. ... This article is about the study of the past in human terms. ... The past is the portion of the timeline that has already occurred; it is the opposite of the future. ... Present redirects here. ... For other uses, see Future (disambiguation). ... Futurology is the detailed critical inspection and reasoning of the state in which things will develop in the future on the basis of existing circumstances in history. ... Image File history File links Portal. ... A time scale specifies divisions of time. ... Wikipedia does not yet have an article with this exact name. ... UTC redirects here. ... Universal Time (UT) is a timescale based on the rotation of the Earth. ... International Atomic Time (TAI, from the French name Temps Atomique International) is a high-precision atomic time standard that tracks proper time on Earths geoid. ... This article is about the unit of time. ... A minute is a unit of time equal to 1/60th of an hour and to 60 seconds. ... The hour (symbol: h) is a unit of time. ... Sidereal time is time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical with, the motion of stars. ... Solar time is based on the idea that when the sun reaches its highest point in the sky, it is noon. ... Timezone and TimeZone redirect here. ... For other uses, see Clock (disambiguation). ... Horology is the study of the science and art of timekeeping devices. ... An astrarium, also called a planetarium, is the mechanical representation of the cyclic nature of astronomical objects in one timepiece. ... A marine chronometer is a timekeeper precise enough to be used as a portable time standard, used to determine longitude by means of celestial navigation. ... For other uses, see Sundial (disambiguation). ... A water clock or clepsydra is a device for measuring time by letting water regularly flow out of a container usually by a tiny aperture. ... Calendar · Day · Week · Month · Year · Tropical year · Julian · Gregorian · Islamic For other uses, see Calendar (disambiguation) A page from the Hindu calendar 1871–1872. ... Look up day in Wiktionary, the free dictionary. ... For more details on each day of the week, see days of the week. ... Look up Month in Wiktionary, the free dictionary. ... A year (from Old English gÄ“r) is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ... A tropical year is the length of time that the Sun, as viewed from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere). ... The Julian calendar was a reform of the Roman calendar which was introduced by Julius Caesar in 46 BC and came into force in 45 BC (709 ab urbe condita). ... For the calendar of religious holidays and periods, see liturgical year. ... The Islamic calendar or Muslim calendar (Arabic: التقويم الهجري; at-taqwÄ«m al-hijrÄ«; Persian: تقویم هجري قمری ‎ taqwÄ«m-e hejri-ye qamari; also called the Hijri calendar) is the calendar used to date events in many predominantly Muslim countries, and used by Muslims everywhere to determine the proper day on which to celebrate...   Intercalation · Leap second · Leap year
Chronology	Astronomical chronology · Geologic Time · Geological history · Geochronology · Archaeological dating Calendar era · Regnal year · Chronicle · Timeline · Periodization
Religion and Mythology	Time and fate deities · Wheel of time · Kāla · Kalachakra · Prophecy · Dreamtime
Philosophy	Causality · Eternalism · Eternal return · Event · The Unreality of Time · A-series and B-series · B-Theory of time Endurantism · Four dimensionalism · Perdurantism · Presentism · Temporal finitism · Temporal parts
Physical Sciences	Time in physics · Spacetime · Absolute time and space · T-symmetry Arrow of time · Chronon · Fourth dimension · Planck epoch · Planck time · Time domain Intercalation is the insertioffn of an extra day, week or month into some calendar years to make the calendar follow the seasons. ... A leap second is a one-second adjustment to civil time in order to keep it close to the mean solar time. ... For the novel by Michael Crichton, see Timeline (novel). ... // For other uses, see time scale. ... Geological time scale. ... Geochronology is the science of determining the age of rocks, fossils, and sediments. ... Dating material drawn from the archaeological record can made by a direct study of a artifact or may be deduced by association with materials found in the context the item is drawn from or inferred by its point of discovery in the sequence relative to datable contexts. ... A calendar era is the year numbering system used by a calendar. ... Regnal year: the year of the reign of a sovereign. ... Generally a chronicle (Latin chronica, from Greek Χρόνος) is historical account of facts and events in chronological order. ... Look up timeline in Wiktionary, the free dictionary. ... Periodization is the attempt to categorize or divide time into discrete named blocks. ... Wheel of time may refer to: The Wheel of time or history, a religious concept predominant in Buddhism and Hinduism The Wheel of Time, a fantasy book series by author Robert Jordan The Wheel of Time (computer game), an action first-person shooter based on the series The Timewheel, a... is the Sanskrit for time (from a root to enumerate; unrelated to black whence ). It denotes a fixed or right point in time (compare rtu, kairos). ... Kālacakra (Sanskrit कालचक्र; Tibetan དུས་ཀྱི་འཁོར་ལོ་ dus kyi khor lo) is a term used in Tantric Buddhism that means time-wheel or time-cycles. It refers both to a Tantric deity (Tib. ... For other uses, see Prophecy (disambiguation). ... opens chapter nine of The Dreaming Universe (1994) entitled The Dreamtime with a quote from The Last Wave, a film by Peter Weir: Aboriginals believe in two forms of time. ... Philosophy of space and time is the branch of philosophy concerned with the issues surrounding the ontology, epistemology, and character of space and time. ... Causality or causation denotes the relationship between one event (called cause) and another event (called effect) which is the consequence (result) of the first. ... Eternalism is a philosophical approach to the ontological nature of time. ... Eternal return or sometimes eternal recurrence is a concept originating from ancient Egypt and developed in the teachings of Pythagoras. ... This page is a candidate for speedy deletion because: this page is a test If you disagree with its speedy deletion, please explain why on its talk page or at Wikipedia:Speedy deletions. ... Wikisource has original text related to this article: The Unreality of Time To meet Wikipedias quality standards, this article or section may require cleanup. ... A-series and B-series are terms introduced by the Scottish idealist philosopher John McTaggart in 1908 which have become classic terms of reference in modern discussions of the philosophy of time, even outside the analytic tradition. ... The B-theory of time is a term, given to one a two positions taken by theorists, in the philosophy of time. ... This article or section is in need of attention from an expert on the subject. ... In the philosophy of time, four dimensionalism is the view that reality is a four-dimensional continuum composed of time and space (spacetime). ... Perdurantism or perdurance theory is a philosophical theory of persistence and identity. ... In the philosophy of time, presentism is the belief that neither the future nor the past exists. ... Temporal Parts are used in contemporary metaphysics in the debate over persistence of material objects. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... For other uses of this term, see Spacetime (disambiguation). ... In physics, the concept of absolute time and absolute space are hypothetical models in which time either runs at the same rate for all the observers in the universe or the rate of time of each observer can be scaled to the absolute time by multiplying the rate by a... T-symmetry is the symmetry of physical laws under a time-reversal transformation— The universe is not symmetric under time reversal, although in restricted contexts one may find this symmetry. ... This article or section does not cite its references or sources. ... A chronon is a hypothetical concept in theoretical physics intended to describe a quantum of time. ... For other uses, see Fourth dimension (disambiguation). ... Named after Max Planck, in cosmology the Planck epoch (or Planck Era) is the earliest period of time in the history of the universe, from zero to 10-43 seconds (one Planck time), during which all four fundamental forces were unified and elementary particles did not yet exist. ... In physics, the Planck time (tP), is the unit of time in the system of natural units known as Planck units. ... Time-domain is a term used to describe the analysis of mathematical functions, or real-life signals, with respect to time. ... Theory of relativity · Time dilation · Gravitational time dilation · Coordinate time · Proper time
Psychology	Consciousness and spacetime · Mental chronometry · Reaction time · Sense of time · Specious present
Sociology and Anthropology	Futures studies · Long Now Foundation · Time discipline
Economics	Newtonian time in economics · Time value of money
Related topics	Space · Duration · Time capsule · Time travel · Time signature · System time · Carpe diem · Tempus fugit

v • d • e Time measurement and standards
Major subjects	Time · Chronometry · Orders of magnitude · Units of time · Metrology	Time Portal
International standards	UTC · UT · TAI · ISO 31-1 · Second · DUT1 · Leap second · IERS Terrestrial Time · Geocentric Coordinate Time · Barycentric Coordinate Time Two-dimensional analogy of space-time curvature described in General Relativity. ... Time dilation is the phenomenon whereby an observer finds that anothers clock which is physically identical to their own is ticking at a slower rate as measured by their own clock. ... Gravitational time dilation is a consequence of Albert Einsteins theories of relativity and related theories which causes time to pass at different rates in regions of a different gravitational potential; the higher the local distortion of spacetime due to gravity, the slower time passes. ... Coordinate time is the interval of time independent of relativistic time dilation. ... In relativity, proper time is time measured by a single clock between events that occur at the same place as the clock. ... Space-time theories of consciousness relate the geometrical features of conscious experience, such as viewing things in space-time at a point, to the geometrical properties of the universe itself. ... // Definition and history Psychologists have investigated mental chronometry for over 100 years. ... Reaction time (RT) is the elapsed time between the presentation of a sensory stimulus and the subsequent behavioral response. ... Although the sense of time is not associated with a specific sensory system, the work of psychologists and neuroscientists indicates that our brains do have a system governing the perception of time. ... The specious present is the time duration wherein ones perceptions are considered to be in the present. ... Future studies reflects on how today’s changes (or the lack thereof) become tomorrow’s reality. ... The Long Now Foundation, established in 1996, is a private organization that seeks to become the seed of a very long-term cultural institution. ... In sociology and anthropology, time discipline is the general name given to social and economic rules, conventions, customs, and expectations governing the measurement of time, the social currency and awareness of time measurements, and peoples expectations concerning the observance of these customs by others. ... The time value of money is the premise that an investor prefers to receive a payment of a fixed amount of money today, rather than an equal amount in the future, all else being equal. ... This article is about the idea of space. ... A duration is an amount of time or a particular time interval. ... For other uses, see Time capsule (disambiguation). ... Time travel is a concept that has long fascinated humanity—whether it is Merlin experiencing time backwards, or religious traditions like Mohammeds trip to Jerusalem and ascent to heaven, returning before a glass knocked over had spilt its contents. ... The time signature (also known as meter signature) is a notational convention used in Western musical notation to specify how many beats are in each measure and what note value constitutes one beat. ... In computer science and computer programming, system time represents a computer systems notion of the passing of time. ... For other uses, see Carpe diem (disambiguation). ... Tempus fugit on a sundial Tempus fugit is a Latin expression meaning time flees, more commonly translated as time flies. It is frequently used as an inscription on clocks. ... A time scale specifies divisions of time. ... This article is about the concept of time. ... Wikipedia does not yet have an article with this exact name. ... // The pages linked in the right-hand column contain lists of times that are of the same order of magnitude (power of ten). ... Metrology (from Greek metron (measure), and -logy) is the science of measurement. ... Image File history File links Portal. ... UTC redirects here. ... Universal Time (UT) is a timescale based on the rotation of the Earth. ... International Atomic Time (TAI, from the French name Temps Atomique International) is a high-precision atomic time standard that tracks proper time on Earths geoid. ... ISO 31-1 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to space and time. ... This article is about the unit of time. ... The time correction DUT1 is the difference between the Universal Time scale UT1 (which corresponds to the Earths rotation) and the Coordinated Universal Time (UTC) (which corresponds, except for a constant offset, to International Atomic Time (TAI)). DUT1 = UT1 - UTC DUT1 is maintained in the range -0. ... A leap second is a one-second adjustment to civil time in order to keep it close to the mean solar time. ... The International Earth Rotation and Reference Systems Service is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Parameter (EOP) and International Celestial Reference System (ICRS) groups. ... Terrestrial Time (TT) is the modern time standard for time on the surface of the Earth. ... Geocentric Coordinate Time (TCG) is a coordinate time standard intended to be used as the independent variable of time for all calculations pertaining to precession, nutation, the Moon, and artificial satellites of the Earth. ... Barycentric Coordinate Time (TCB) was defined in 1991 by the International Astronomical Union as one of the replacements for the ill-defined Barycentric Dynamical Time (TDB). ... Civil time · Minute · Hour · 12-hour clock · 24-hour clock · ISO 8601 Civil time is another name for mean solar time reckoned from midnight. ... A minute is a unit of time equal to 1/60th of an hour and to 60 seconds. ... The hour (symbol: h) is a unit of time. ... The 12-hour clock is a timekeeping convention in which the 24 hours of the day are divided into two periods called ante meridiem (a. ... The 24-hour clock is a convention of time keeping in which the day runs from midnight to midnight and is divided into 24 hours, numbered from 0 to 23. ... ISO 8601 is an international standard for date and time representations issued by the International Organization for Standardization (ISO). ... International Date Line · Solar time · Time zone · Daylight saving time · Time offset · Annum
Obsolete standards	Ephemeris time · Barycentric Dynamical Time · Greenwich Mean Time · Prime Meridian
Time in physics	Spacetime · Chronon · Cosmological decade · Planck epoch · Planck time · T-symmetry Theory of relativity · Time dilation · Gravitational time dilation · Coordinate time · Proper time “Date line” redirects here. ... Solar time is based on the idea that when the sun reaches its highest point in the sky, it is noon. ... Timezone and TimeZone redirect here. ... Although DST is common in Europe and North America, most of the worlds people do not use it. ... A time offset is defined by international convention as a number of hours and minutes from Coordinated Universal Time in Greenwich, England. ... Annum is a Latin noun meaning year. ... Ephemeris Time (ET) is a now obsolete time scale used in ephemerides of celestial bodies, in particular the Sun (as observed from the Earth), Moon, planets, and other members of the solar system. ... Barycentric Dynamical Time (TDB) was defined by the International Astronomical Union (IAU) in 1976 to be used as the relativistic replacement for the non-relativistic Ephemeris Time which had been used in the ephemerides starting in 1960. ... GMT redirects here. ... Location of the Prime Meridian Image:Prime Meridian. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... For other uses of this term, see Spacetime (disambiguation). ... A chronon is a hypothetical concept in theoretical physics intended to describe a quantum of time. ... A cosmological decade ( CÐ ) is a division of the lifetime of the cosmos. ... Named after Max Planck, in cosmology the Planck epoch (or Planck Era) is the earliest period of time in the history of the universe, from zero to 10-43 seconds (one Planck time), during which all four fundamental forces were unified and elementary particles did not yet exist. ... In physics, the Planck time (tP), is the unit of time in the system of natural units known as Planck units. ... T-symmetry is the symmetry of physical laws under a time-reversal transformation— The universe is not symmetric under time reversal, although in restricted contexts one may find this symmetry. ... Two-dimensional analogy of space-time curvature described in General Relativity. ... Time dilation is the phenomenon whereby an observer finds that anothers clock which is physically identical to their own is ticking at a slower rate as measured by their own clock. ... Gravitational time dilation is a consequence of Albert Einsteins theories of relativity and related theories which causes time to pass at different rates in regions of a different gravitational potential; the higher the local distortion of spacetime due to gravity, the slower time passes. ... Coordinate time is the interval of time independent of relativistic time dilation. ... In relativity, proper time is time measured by a single clock between events that occur at the same place as the clock. ... Time domain · Continuous time · Discrete time · Absolute time and space
Horology	Clock · Astrarium · Atomic clock · Hourglass · Marine chronometer · Radio clock · Sundial · Water clock History of timekeeping devices · Equation of time · Complication
Calendar	Day · Week · Month · Year · Decade · Century · Millennium Astronomical · Julian · Gregorian · Islamic · Lunisolar · Solar · Lunar · Epact · Intercalation · Leap year Time-domain is a term used to describe the analysis of mathematical functions, or real-life signals, with respect to time. ... A continuous signal or a continuous time signal is a varying quantity (a signal) that can be, or is expressed, as a continuous function of an independent variable, usually time. ... Discrete time is non-continuous time. ... In physics, the concept of absolute time and absolute space are hypothetical models in which time either runs at the same rate for all the observers in the universe or the rate of time of each observer can be scaled to the absolute time by multiplying the rate by a... Horology is the study of the science and art of timekeeping devices. ... For other uses, see Clock (disambiguation). ... An astrarium, also called a planetarium, is the mechanical representation of the cyclic nature of astronomical objects in one timepiece. ... “Nuclear Clock” redirects here. ... For other uses, see Hourglass (disambiguation). ... A marine chronometer is a timekeeper precise enough to be used as a portable time standard, used to determine longitude by means of celestial navigation. ... A radio clock A radio clock is a clock that is synchronized by a time code bit stream transmitted by a radio transmitter connected to a time standard such as an atomic clock. ... For other uses, see Sundial (disambiguation). ... A water clock or clepsydra is a device for measuring time by letting water regularly flow out of a container usually by a tiny aperture. ... The equation of time is the difference, over the course of a year, between time as read from a sundial and a clock. ... In horology terms, a complication in a mechanical timepiece is any feature beyond that of a simple hours, minutes, and seconds movement. ... For other uses, see Calendar (disambiguation) A page from the Hindu calendar 1871–1872. ... Look up day in Wiktionary, the free dictionary. ... For more details on each day of the week, see days of the week. ... Look up Month in Wiktionary, the free dictionary. ... A year (from Old English gÄ“r) is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ... A decade is a set or a group of ten, commonly a period of 10 years in contemporary English, or a period of 10 days in the French revolutionary calendar. ... A century (From the Latin cent, one hundred) is one hundred consecutive years. ... A millennium (pl. ... Astronomical year numbering is based on BCE/CE (or BC/AD) year numbering, but follows normal decimal integer numbering more strictly. ... The Julian calendar was a reform of the Roman calendar which was introduced by Julius Caesar in 46 BC and came into force in 45 BC (709 ab urbe condita). ... For the calendar of religious holidays and periods, see liturgical year. ... The Islamic calendar or Muslim calendar (Arabic: التقويم الهجري; at-taqwÄ«m al-hijrÄ«; Persian: تقویم هجري قمری ‎ taqwÄ«m-e hejri-ye qamari; also called the Hijri calendar) is the calendar used to date events in many predominantly Muslim countries, and used by Muslims everywhere to determine the proper day on which to celebrate... A lunisolar calendar is a calendar whose date indicates both the moon phase and the time of the solar year. ... A solar calendar is a calendar whose dates indicate the position of the earth on its revolution around the sun (or equivalently the apparent position of the sun moving on the celestial sphere). ... A lunar calendar is a calendar that is based on cycles of the moon phase. ... The epact (from Greek: epaktai hèmerai = added days) is, as the second Canon of the Gregorian Calendar reform puts it, nothing else than the number of days which the common solar year of 365 days surpasses the common lunar year of 354 days (Latin: Epacta nihil aliud est quam... Intercalation is the insertioffn of an extra day, week or month into some calendar years to make the calendar follow the seasons. ... Tropical year · Equinox · Solstice · Days of the week · Calculating the day of the week · Dominical letter
Archaeology & geology	International Commission on Stratigraphy · Geologic Time · Archaeological dating
Astronomical chronology	Nuclear time scale · Precession · Sidereal time · Galactic year
Unusual units	FFF System · Fortnight · Jiffy · Lustrum · Saeculum · Shake · Tide
Related topics	Chronology · Duration · System time · Mental chronometry · Time value of money