Because it is a solar and lunar calendar (the year follows the seasons and the months keep track of the phases of the moon), the Jewish calendar has an extra leap month 7 times every 19 years. This leap month occurs in spring, and is the month preceding Passover. It is therefore appropriate to count the Jewish months starting with Nissan, the month of Passover, as follows:
Since the Jewish year begins in the fall, this numeration of months will not always follow an increasing order; that is, later dates in the same year may have a smaller number for the month:
For this reason, some calendar functions will still work with the
Calendar->Jewish option, but will not follow the same rules.
DaysBetween can return a positive number even if the second date is lexicographically smaller (smaller in the canonical
Mathematica ordering):
DaysPlus can return a lexicographically smaller (smaller in the canonical
Mathematica ordering) result for a later date: