Hebrew birthday calculator

A Hebrew month is one cycle of the moon, 29 or 30 days. Twelve of them come to 353, 354 or 355 days, about eleven days short of the solar year, so a Hebrew birthday slides earlier against the Gregorian calendar every year.

It does not slide away entirely. Seven years in every nineteen add a thirteenth month, Adar I, which pushes the date back by roughly a month and keeps Passover in spring. The net effect is a birthday that wanders across a window of a few weeks rather than drifting through the seasons the way an Islamic date does.

That is also why the table above sometimes carries a note. A date in Adar, or the 30th of a month that has only 29 days in a given year, has no exact equivalent, and the rule that settles it is stated in the row rather than applied quietly.

A boy reaches bar mitzvah on his thirteenth Hebrew birthday and a girl bat mitzvah on her twelfth in most communities. That date is fixed by the Hebrew calendar, not by the party: the obligations begin then whether anything is held or not.

The celebration is usually put on a Shabbat near the date, which is why families end up working backwards from a Torah reading and a hall rather than forwards from the birthday. The calculator above gives the Hebrew date and its Gregorian equivalent, so you can see which Shabbatot sit either side of it.

The same Hebrew date is what a family uses a generation later, for a grandchild named after the same person. It is the one date that stays put while the civil one moves around it.