I thought it was a Lunar Calendar from the time of Babylon that the Romans also used. Now I got something new to study, thank you.In addition, post exile the Temple used the Babylonian calendar while the Essenes maintained the lunar calendar as used by Moses.
This accounts for many “discrepancies” in scripture including Jesus account of eating Passover, then being crucified on Passover. In short, he ate Passover on the Lunar calendar and was crucified on the Babylonian calendar.