Two calendars, one day
Why the two dates move apart
A family day kept on the Hebrew calendar lands on a different Gregorian date every year. Nothing is wrong: the two calendars count time in different ways, and LuachCal follows the Hebrew one.
About eleven days a year
The Hebrew year follows the moon: twelve months of 29 or 30 days, about 354 days in all. The Gregorian year follows the sun and has 365. So in an ordinary year a Hebrew date arrives about eleven days earlier on the Gregorian calendar than it did the year before.
A thirteenth month, seven years in nineteen
To keep the festivals in their seasons, seven years in every nineteen add a whole month, Adar I, before Adar. In those years the Hebrew date lands about nineteen days later instead of eleven earlier. After nineteen years the two calendars nearly meet again, which is why a Hebrew birthday often falls on the Gregorian birthday at 19, 38 and 57.
The Hebrew day begins at nightfall
A Hebrew day starts the evening before, not at midnight. Someone born at nine at night on 3 March was born on the next Hebrew day. That is why LuachCal asks whether a Gregorian date was after nightfall, and does not ask when you give a Hebrew date.
Adar, in a year that has two
When the original day was in Adar of an ordinary year, a leap year offers two Adars. A birthday, and a bar or bat mitzvah, is kept in Adar II. A yahrzeit is kept in Adar I by the common Ashkenazi custom and in Adar II by the Sephardi one. When your family’s custom changes the day, LuachCal asks once and remembers the answer.
Days that are not there every year
Cheshvan and Kislev have 30 days in some years and 29 in others. A yahrzeit on the 30th is kept on the 29th in a short year. A birthday on the 30th moves forward to the 1st of the next month instead.
Bar and bat mitzvah
A bar mitzvah falls on the 13th Hebrew birthday and a bat mitzvah on the 12th. Because they follow the Hebrew calendar, their Gregorian date is rarely the Gregorian birthday.