The proleptic Julian calendar is produced by extending the Julian calendar to dates preceding its official introduction in 45 BC.
Historians since Bede have traditionally represented the years preceding AD 1 as "1 BC", "2 BC", etc. In this system the year 1 BC would be a leap year (although the leap years actually observed between 46 BC and AD 4 were erratic: see the Julian calendar article for details). (Bede and later Latin writers chose not to place the Latin zero, nulla, between BC and AD years.)
To determine an interval in years across the BC/AD boundary, it is more convenient to include a year zero and represent earlier years as negative. This is the convention used in the "astronomical Julian calendar". In this system the year 0 (equivalent to 1 BC) is a leap year.
Likewise, the Proleptic Gregorian Calendar is used to specify dates before its official introduction in 1582. Because the Julian Calendar was actually used before that time, one must explicitly state that a given date is in the Proleptic Gregorian Calendar when that is used.
This Julian in prolepticJuliancalendar and Julian year refers to Julius Caesar, who..., 1582 C.E. in the Gregorian Calendar, which is the date before the day on which the Gregorian...
The proleptic Gregorian calendar is produced by extending the Gregorian Calendar to dates preceding its official introduction.
Likewise, the ProlepticJulianCalendar is used to specify dates before its official introduction in 45 BC.
The Juliancalendar was in general use in Europe from the times of the Roman Empire until 1582, when Pope Gregory XIII promulgated the Gregorian Calendar, which was soon adopted by most Catholic countries.
Russia remained on the Juliancalendar until after the Russian Revolution (which is thus called the 'October Revolution' but occurred in November according to the Gregorian calendar), in 1917, while Greece continued to use it until 1923.
A revised Juliancalendar was proposed during a synod in Constantinople in May of 1923, consisting of a solar part which was and will be identical to the Gregorian calendar until the year 2800, and a lunar part which calculated Easter astronomically at Jerusalem.