[[File:List_of_Babylonian_months_by_the_Chaldeans,_2000_BC_2015-07-24_10-24.jpg|thumbnail|List of Babylonian months by the Chaldeans, 2000 BC]]
The '''Babylonian calendar''' was a [[lunisolar calendar]] with years consisting of 12 [[lunar month]]s, each beginning when a new [[lunar phase|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]]) predecessor preserved in the '''Umma calendar''' of [[Shulgi]] (c. 21st century BC).