In the Babylonian calendar a year consisted of 12 lunar months, each beginning when a new crescent moon was first sighted low on the western horizon at sunset. This system came into use sometime before 2000 BC.

The names of the months were:

• Nisanu
• Ayaru
• Simanu
• Du`uzu
• Abu
• Ululu
• Tashritu
• Arakhsamna
• Kislimu
• Tebetu
• Shabatu
• Adaru

Beginning about 499 BC the calendar began to be regulated by a lunisolar cycle of 19 years equaling 235 months. Although usually called the Metonic cycle, Meton (432 BC) probably learned of the cycle from the Babylonians. After no more than three isolated exceptions, by 380 BC the calendar was regulated by the cycle without exception. Within the cycle of 19 years, the month Adaru 2 was intercalated, except in the year that was number 17 in the cycle, when the month Ululu 2 was inserted. During this period, the first day of each month (beginning at sunset) continued to be the day when a new crescent moon was first sighted—the calendar never used a specified number of days in any month.

See also Hebrew calendar.