What is the tabular Islamic calendar?

The tabular calendar is a fixed arithmetic scheme where 11 of every 30 years are leap years with 355 days instead of 354. It avoids the need for moon sightings and is fully predictable, which makes it ideal for software and historical conversion.

Why might the date differ from the one announced locally?

Religious observance often relies on the actual sighting of the new crescent moon, which can fall a day before or after the arithmetic prediction depending on location and weather. So Ramadan or Eid announced by a mosque may differ from the tabular date by up to a day.

When does the Hijri calendar begin?

The epoch is 1 Muharram of year 1 AH, which in this tabular scheme corresponds to 19 July 622 in the proleptic Gregorian calendar (16 July 622 in the Julian calendar). AH stands for Anno Hegirae, the year of the Hijra.

How long is a Hijri year?

A Hijri year is 12 lunar months totalling 354 or 355 days, about 11 days shorter than a solar year. That is why Islamic dates such as Ramadan move earlier through the Gregorian seasons by roughly 11 days each year.

Are the month lengths fixed?

In the tabular calendar odd-numbered months have 30 days and even-numbered months have 29, except the twelfth month Dhu al-Hijjah which gains a day to 30 in leap years. The true lunar calendar instead lets each month run 29 or 30 days based on the moon.

What is the Gregorian ↔ Hijri Date Converter?

Convert any Gregorian date to the Islamic Hijri calendar and back using the standard tabular (arithmetic) algorithm. Returns the Hijri day, named month and AH year, with a note that moon-sighting dates can vary by a day. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Gregorian ↔ Hijri Date Converter

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

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The Islamic Hijri calendar is purely lunar, so its dates drift through the Gregorian seasons by about 11 days each year. This converter maps dates between the two systems with the widely used tabular (arithmetic) algorithm, which is deterministic and matches the calendars built into most software.

How it works

Both dates are converted through the chronological Julian Day Number (JDN), then into the target calendar:

Gregorian (y,m,d) ─► JDN ─► Hijri (y,m,d)
Hijri     (y,m,d) ─► JDN ─► Gregorian (y,m,d)

The Hijri side uses the standard 30-year cycle in which years 2, 5, 7, 10, 13, 16, 18, 21, 24, 26 and 29 are leap years of 355 days; the rest have 354. Odd months are 30 days, even months 29, with Dhu al-Hijjah taking the extra leap day.

The twelve months of the Hijri calendar

#	Month	Days (ordinary / leap)
1	Muharram	30
2	Safar	29
3	Rabi al-Awwal	30
4	Rabi al-Thani	29
5	Jumada al-Ula	30
6	Jumada al-Akhirah	29
7	Rajab	30
8	Shaban	29
9	Ramadan	30
10	Shawwal	29
11	Dhu al-Qadah	30
12	Dhu al-Hijjah	29 / 30

In leap years Dhu al-Hijjah gains one day, taking the total year length from 354 to 355 days.

Worked examples

1 January 2000 → 24 Ramadan 1420 AH. Ramadan is the ninth month and falls in the middle of winter in northern latitudes in that year.
6 June 2026 → 20 Dhu al-Hijjah 1447 AH, placing the date in the month of Hajj.
1 Muharram 1446 AH → 7 July 2024 in the Gregorian calendar, marking the Islamic New Year that year.

Because the Hijri year is roughly 11 days shorter than the solar year, each Islamic date moves earlier through the Gregorian seasons by about 11 days annually. Over 33 years Ramadan completes a full cycle through all seasons.

Tabular vs. moon-sighted dates

The tabular calendar used here is a mathematical approximation of the true lunar calendar. It is reliable for civil, legal, and historical purposes, and it is what most software libraries and international organisations use. However, religious observance in most Muslim-majority countries — including the start of Ramadan, the date of Eid al-Fitr, and Eid al-Adha — is determined by the actual sighting of the new crescent moon (hilal) by local or national religious authorities.

The result from this tool may therefore differ from the announced date by one day in either direction, depending on location, weather conditions, and the criterion used (naked-eye sighting, telescope-assisted sighting, or calculated visibility). For prayer times and religious observance, always defer to the ruling of the relevant religious authority for your location.

When Gregorian-to-Hijri conversion is useful

Historical documents: dating letters, deeds, or records that use AH years to find the corresponding Gregorian date for archives or genealogy research.
Contract deadlines: some Islamic finance agreements express milestone dates in both calendars; conversion helps cross-check which Gregorian date corresponds to a given Hijri deadline.
Event planning: planning around Ramadan, Hajj season, or public holidays in countries that observe the Islamic calendar alongside the Gregorian one.
Academic research: translating dates in medieval Islamic manuscripts or historical chronicles to a common reference frame.