This article is from the Calendars FAQ, by Claus Tondering firstname.lastname@example.org with numerous contributions by others.
A calculated new moon is used. In order to understand the
calculations, one must know that an hour is subdivided into 1080
The calculations are as follows:
The new moon that started the year AM 1, occurred 5 hours and 204
parts after sunset (i.e. just before midnight on Julian date 6 October
The new moon of any particular year is calculated by extrapolating
from this time, using a synodic month of 29 days 12 hours and 793
Note that 18:00 Jerusalem time (15:39 UTC) is used instead of sunset in
all these calculations.