Hijri Gregorian Date Converter
Convert between the Gregorian and Hijri (Islamic) calendars using the tabular arithmetic calendar.
What does the undefined do?
It converts dates between the Gregorian and Hijri calendars in either direction, using the tabular arithmetic method used in many civil applications.
- Inputs: a direction, and either a Gregorian date or a Hijri year, month and day.
- Output: the date in the other calendar.
- Method: both calendars are converted through a Julian day number, using the standard arithmetic Hijri calendar.
Quick answer
The Gregorian date 4 October 2026 is 21 Rabi al-Thani 1448 in the arithmetic Hijri calendar. Observed dates can differ by a day because religious months begin at moon sighting.
What This Calculator Really Does
The Hijri calendar is lunar, with twelve months of 29 or 30 days and a year about eleven days shorter than the solar year, which is why Islamic dates drift earlier each Gregorian year. Converting between the two is arithmetic: the Gregorian date becomes a Julian day number, then the Hijri date is derived from the Islamic epoch using a 30-year cycle of leap years. This tool uses the tabular arithmetic calendar, the same method used in many civil and software calendars. It does not use sighting data, so a religious date may land one day earlier or later than your local announcement.
The formula it uses
Julian day from the Gregorian date uses the standard civil formula with the century correction. The Hijri date is found from the Julian day minus the Islamic epoch, divided into 30-year cycles and years with the tabular leap-year rule, then the month and day from a fixed month-length pattern.
Worked example with real numbers
4 October 2026 Gregorian converts to 1448-04-21 Hijri. Going the other way, 1 Muharram 1448 converts to 17 June 2026, and 1 Muharram 1447 to 27 June 2025, matching the roughly eleven-day annual drift.
Common mistakes to avoid
- Treating the result as the officially observed date, when moon sighting can shift it by a day.
- Switching the direction select and then reading the wrong date.
- Assuming the Islamic New Year is a fixed Gregorian date, when it moves about eleven days earlier every year.
Assumptions and limitations
The arithmetic (tabular) calendar is used throughout, with a fixed 30-year leap cycle and no local sighting adjustment. Years are given as plain numbers without the AH or CE suffix.
Disclaimer
This is a date arithmetic tool, not a religious authority. For worship and official purposes, confirm the date with your local moon-sighting body, since observed months can begin a day apart from the arithmetic calendar.
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Frequently Asked Questions
How is the undefined calculated?
The Gregorian date is turned into a Julian day number, then shifted into the Islamic epoch and resolved through the tabular 30-year cycle to give the Hijri year, month and day. The reverse direction uses the same arithmetic backwards.
What do I need to use the undefined?
A direction, plus either a Gregorian date or a Hijri year, month and day. No account or app is needed.
What does the result from the undefined show?
The equivalent date in the other calendar, shown with zero-padded month and day for clarity.
Why does the Hijri year move earlier each Gregorian year?
Because the lunar year is about eleven days shorter than the solar year, so the same Hijri date lands about eleven days earlier on the Gregorian calendar every year.
Is this the same date my mosque will announce?
Usually within a day. This tool uses the arithmetic calendar, while mosques often follow actual moon sighting, which can begin a month a day earlier or later.
Is the undefined really free?
Yes — 100 percent free, no sign-up, and everything runs in your browser.