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The Babylonian calendar was a lunisolar calendar with years consisting of 12 lunar months, each beginning when a new crescent moon was first sighted low on the western horizon at sunset, plus an intercalary month inserted as needed by decree. The calendar is based on a Sumerian (Ur III) precedecessor preserved in the Umma calendar of Shulgi (ca. 21st century BC).

The year begins in spring, and is divided into reš šatti "beginning", mišil šatti "middle", and kît šatti "end of the year". The name for "month" was arxu (status constructus arax). That the calendar originates in Babylonian, not Assyrian times is shown by the fact that the chief deity of the Assyrians is assigned the surplus intercalary month. During the 6th century BC Babylonian exile of the Hebrews, the Babylonian month names were adopted into the Hebrew calendar.

The names of the months were:

1. Arax Nisânu, Sumerian ITU BARAG.ZAG(.GAR) "month of the sanctuary", zodiacal sign KU (Aries) , dedicated to Anu and Bel
2. Arax Âru, Sumerian ITU GUDDA.SIDI "month of the righteous bull", dedicated to Ea
3. Arax Simanu, dedicated to Sin, zodiacal sign BI(KAŠ) Gemini
4. Arax Du'uzu (DU.ZID, Dumuzi), dedicated to Adar
5. Abu, Sumerian NE.NE.GAR, zodiacal sign A (âru, Leo)
6. Ulûlu, dedicated to Ishtar
7. Tiš-ri-tum, the Piel of šurru "to begin", viz., "beginning of the second half-year", dedicated to Shamash. Sumerian ITU DU.U.AZAG
8. Arax-samna, Palrymenian Kanûn, Sumerian ITU APIN.GAB(.BA) "month of laying foundations", dedicated to Marduk, associated with Scorpio
9. Arax Kislimu, Sumerian ITU KAN.KAN.NA, dedicated to Nergal, associated with Sagittarius
10. Arax Ṭebêtum, Sumerian ITU AB.BA.UD.DU "month of the forthcoming of water", dedicated to Pap-sukkal, zodiacal sign sax "ibex" (Capricorn?)
11. Arax Šabaṭu, Sumerian ITU AŠ.A.AN, dedicated to Ramman, zodiacal sign GU qâ (Aquarius?)
12. Addaru, Adâr, Sumerian ITU ŠE.KIN.DUD, dedicated to Dibbara the god of pestilence. In this month, Marduk is called "the fish of Ea", and the constellation/sign of Pisces may derive its name from this.
13. The inter-calary arax makaruša Addari (Ve-adâr), dedicated to Ashur

Until the fifth century BC the calendar was fully observational, but beginning about 499 BC the months 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 months of the calendar were 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.

• Parker, Richard A. and Waldo H. Dubberstein. Babylonian Chronology 626 B.C.-A.D. 75. Providence, RI: Brown University Press, 1956.
• Babylonian and Seleucid Dates 331-29 BC
• W. Muss-Arnolt, The Names of the Assyro-Babylonian Months and Their Regents, Journal of Biblical Literature (1892).
• Sacha Stern, "The Babylonian Calendar at Elephantine" in Zeitschrift für Papyrologie und Epigraphik 130 (2000) 159-171 (PDF document, 94KB)]

• Hebrew calendar
• Assyrian calendar
• Zoroastrian calendar