Hebrew birthday calculator / Adar and the 30th
Born in Adar, or on the 30th of a month?
Some Hebrew dates don't exist every year. Here's what the calculator does in those years, what the sources say, and where the decision belongs to your rabbi.
Confirm with your rabbi. This page explains calendar rules and quotes the sources named below. Customs differ between communities, and the date for a bar or bat mitzvah is your rabbi's call, not a website's.
| Born on | In a regular year | In a leap year | What the sources say |
|---|---|---|---|
| Adar, in a regular year | Adar | Adar II | Chabad.org: bar mitzvah in Adar II; some authorities hold Adar I. Some people mark the birthday in both. |
| Adar I, in a leap year | Adar | Adar I | Chabad.org: one born in a leap year keeps the month he was born in. |
| Adar II, in a leap year | Adar | Adar II | Always the last month of the year. |
| 30 Cheshvan | 30 Cheshvan, or 1 Kislev when there is no 30th | Same | Chabad.org gives 1 Kislev for a bar mitzvah in a year without 30 Cheshvan. |
| 30 Kislev | 30 Kislev, or 1 Tevet when there is no 30th | Same | Chabad.org gives 1 Tevet for a bar mitzvah in a year without 30 Kislev. |
| 30 Adar I | 1 Nisan (Hebcal's list) | 30 Adar I | No source we checked gives the bar mitzvah date for this case. Ask your rabbi. |
Sources: Hebcal developer docs, HebrewCalendar.getBirthdayOrAnniversary (opens in a new tab) (which cites Reingold and Dershowitz, Calendrical Calculations); Chabad.org, "The Laws of Bar Mitzvah" (opens in a new tab); Chabad.org, "When Do I Celebrate My Birthday in a Leap Year?" (opens in a new tab) · checked 4 Oct 2026
Why some dates go missing
Cheshvan and Kislev have 29 days in some years and 30 in others, so 30 Cheshvan and 30 Kislev only exist in some years. Adar I exists only in leap years, so 30 Adar I is missing in every regular year. See the Hebrew calendar, explained.
What the calculator does
- The 20-year list uses Hebcal's birthday rule: someone born on the 30th of Cheshvan, Kislev or Adar I has the birthday moved to the first of the following month in years without that day. Someone born in Adar of a regular year, or in Adar II, always has it in the last month of the year.
- The bar or bat mitzvah date is the same Hebrew date 13 or 12 years on. When that year has no such day, the calculator shows 1 Kislev or 1 Tevet for 30 Cheshvan or 30 Kislev, as Chabad.org's summary gives, and asks you to check with your rabbi for 30 Adar I.
- Every result in these cases carries a short note saying the same thing.
Birthdays in leap years
Chabad.org notes that for birthday customs (not the bar mitzvah itself), someone born in Adar of a regular year can observe them in both Adar I and Adar II. See Hebrew birthday customs.
Work out your own dates
The Hebrew date, the bar or bat mitzvah date, the parsha and the next 20 Hebrew birthdays, from one English date.
