Why does Tishrei have month number 7 instead of 1?

The Hebrew year for counting purposes begins in Tishrei (around September), but the months are traditionally numbered from Nisan in spring, which the Torah calls the first month. So Nisan is month 1 and Tishrei is month 7, even though the year number increments at Tishrei.

How does the converter handle leap years?

Hebrew leap years, which occur 7 times in a 19-year cycle, add a thirteenth month. In those years the month otherwise called Adar splits into Adar I (month 12) and Adar II (month 13). The tool detects leap years automatically and labels the months accordingly.

What are the postponement rules?

The date of Rosh Hashana can be pushed back by one or two days to prevent certain holidays falling on awkward weekdays. The converter applies all the standard dechiyot rules through the molad calculation, so Rosh Hashana always lands on the canonically correct day.

Why do Hebrew years have different lengths?

A common year can be 353, 354 or 355 days and a leap year 383, 384 or 385 days. The variation comes from whether Cheshvan and Kislev have 29 or 30 days, which is determined so the calendar stays aligned with both moon and sun and obeys the postponement rules.

Is the day counted from sunset?

In Jewish practice the day begins at sunset the previous evening. This converter maps whole civil dates to whole Hebrew dates, so for an event after nightfall you may want the next Hebrew date. The conversion itself is calendar-accurate for the daytime portion of the date.

What is the Gregorian ↔ Hebrew Date Converter?

Convert any Gregorian date to the Hebrew lunisolar calendar and back, with correct month names including Adar I and Adar II in leap years. Uses the standard molad-based arithmetic with all four postponement rules. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Gregorian ↔ Hebrew Date Converter

Name: Gregorian ↔ Hebrew Date Converter
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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The Hebrew calendar is lunisolar: months follow the moon while leap months keep the year aligned with the sun. This converter translates a date between the Gregorian and Hebrew systems using the same molad-based arithmetic and postponement rules that fix the calendar, so the results match printed Jewish calendars exactly.

How it works

Both dates are converted to a common day count (a fixed-day or Rata Die number), then back into the target calendar:

1. Find the molad-based count of months elapsed to the target year
2. Derive the day of 1 Tishrei, then apply the postponement rules
3. Add month lengths (Cheshvan/Kislev variable, Adar split in leap years)
4. The result is the fixed day; convert it to the other calendar

Leap years are identified by the rule that year y is a leap year when (7y + 1) mod 19 is less than 7 — that places 7 leap years in each 19-year cycle.

The four postponement rules (dechiyot)

The Hebrew calendar cannot simply start on the day of the calculated molad (new moon) each year. Four postponement rules can push Rosh Hashanah forward by one or two days:

Dechiyah Molad Zaqen — if the molad falls at or after noon, Rosh Hashanah is pushed to the next day (because the moon’s “birth” is too late to be observed that day).
Dechiyah Lo ADU Rosh — Rosh Hashanah cannot fall on Sunday, Wednesday, or Friday. If the calculation gives those days, the new year advances one day.
Dechiyah Gutradd — corrects for common years that would be too long.
Dechiyah Betutakpat — corrects for leap years that would be too short.

These rules interact, which is why the full arithmetic is non-trivial and why a converter that implements all four produces results matching a printed Jewish calendar, while a simple molad calculation alone does not.

Conversion examples

Gregorian date	Hebrew date	Significance
2025-09-23	1 Tishrei 5786	Rosh Hashanah 5786
1948-05-14	5 Iyyar 5708	Israeli Declaration of Independence
2026-04-02	14 Nisan 5786	Erev Passover 5786

For a birthday or anniversary, converting to Hebrew gives the recurring observance date on the Hebrew calendar, which falls on a different Gregorian date every year.

Working with leap years (Adar I and Adar II)

In a leap year, the single month of Adar is replaced by two months: Adar I (30 days) and Adar II (29 days). Purim always falls in Adar II, so it stays in late winter. When converting a Hebrew-to-Gregorian date in a leap year:

Month 12 is Adar I
Month 13 is Adar II

When converting from Gregorian to Hebrew in a common year, there is only one Adar (month 12). The converter detects the year type automatically and labels the months correctly.

Tips and limitations

The Hebrew day begins at sunset, so for events that happen in the evening the Hebrew date is one day ahead of the Gregorian date. For example, a Friday evening Shabbat dinner falls on a Hebrew Saturday. This converter maps whole civil dates to whole Hebrew dates without adjusting for time of day; for events near sunset, check whether the next Hebrew date applies.