The observations of the astrologers, which were meticulously recorded on a nightly basis over many centuries, led to accurate predictions of various astronomical phenomena and the correct calculation of the solar and lunar year. The Babylonian calendar was based upon the lunar year but, thanks to the astrologer's knowledge, could be reconciled with the solar year by means of intercalary months.
We owe much of our calendar system to the Babylonians. They were probably the first people after the Sumerians to have a calendar. This calendar was very important because without it agriculture could not be planned properly.
There were twelve lunar months in the year but as the months were shorter than our months often an extra month would have to be added. This was called the second Elul. Each week was divided into seven days. The day was divided into six parts each of two hours duration and containing thirty parts. The Babylonians measured time with a water or sun clock.
One can see from this that the Babylonian calendar has marked similarities with our own: for instance the twelve months in the year and seven days in a week.
The year started at the vernal (spring) equinox. The Babylonians, by the time of Hammurapi had names for each month: