Four calendars, one day number, no guesswork
Every calendar conversion is really one conversion performed twice: the date you enter becomes a Julian Day Number, the continuous count of days astronomers use, and that count becomes a date in each target calendar. This page converts between the Gregorian, Julian, Hebrew and Islamic (arithmetical, tabular civil) calendars by their documented rules, and shows the weekday and the day number alongside, so any result can be checked against independent references. Two honesty notes stand at the door: the Islamic calendar as actually observed is set by moon sighting and official announcements, so the tabular dates here can differ from observed religious dates by a day or two; and historical dates before a region adopted the Gregorian calendar carry the usual proleptic conventions, which the method section states.
Disclaimer: This calculator gives estimates for general information only. It is not financial, medical or other professional advice. Islamic dates are the arithmetical (tabular civil) calendar; observed religious dates are set by moon sighting and local authority and may differ by a day or two.
What this calculator does
It converts a date entered in any of the four supported calendars into the same day in all four, with the weekday and the Julian Day Number as the audit trail. The Gregorian and Julian calendars differ by a slow drift (13 days in the 1900s and 2000s, 14 from March 2100), the Hebrew calendar is a computed lunisolar calendar whose month lengths vary by year type, and the Islamic calendar here is the arithmetical tabular version: 354-day years with 11 leap years of 355 in each 30-year cycle, alternating 30 and 29-day months. All four are exact rule systems, so conversions between them are exact within the rules, and the only uncertainty on the page is the one labelled: real-world Islamic observance, which follows sighting and announcement rather than the table.
How to use it
- Historical dates need the calendar of their record. An English date from 1700 is Julian (Britain switched in 1752); a Russian date from 1917 is Julian until February 1918. Enter the date in the calendar it was written in, and read the Gregorian line as its modern equivalent, which is what the worked example does with the October Revolution.
- Hebrew months run from Tishri. The Hebrew year begins in Tishri (autumn), so a Hebrew date converts to a Gregorian date in the autumn of year-minus-3761 or the following seasons. In leap years there are two Adars; Adar II is the one followed by Nisan, and Purim falls in Adar II in leap years and in Adar in common years.
- Islamic observance may differ. For religious purposes, the date that governs is the one announced by the relevant authority for your place, based on the new moon’s sighting. The tabular calendar is the right tool for conversion, records and planning; it is not a fatwa, and the results row says so wherever Ramadan or a new year appears.
- The weekday is a free check. Weekdays are calendar-independent (the seven-day week never reset through any reform), so if you know the weekday a historical event fell on, it verifies the conversion, and famous dates below are given with theirs.
- Proleptic means extended by rule. Gregorian dates before October 1582 and Hebrew or Islamic dates projected before their epochs are the calendars’ rules extended backwards; contemporaries did not use them, and historians’ conventions vary, which the limitations section states.
How it is calculated
- Pivot: each calendar converts to and from the Julian Day Number by its standard published algorithm (Fliegel and Van Flandern form for Gregorian and Hebrew arithmetic after Dershowitz and Reingold’s calendrical calculations; Meeus’s form for the Julian calendar; the standard arithmetical scheme for the tabular Islamic calendar, civil epoch 16 July 622 Julian). Weekday is the day number modulo 7, the week being unbroken.
- Hebrew rules implemented in full: the 19-year cycle with leap years at positions 3, 6, 8, 11, 14, 17 and 19; month counting from the epoch molad with the four postponement rules (dehiyyot); year lengths 353, 354 or 355 days (383, 384 or 385 in leap years) setting Heshvan and Kislev lengths.
- Validation: the implementation reproduces 1 January 2000 as 23 Tevet 5760 and 24 Ramadan 1420, 14 May 1948 as 5 Iyar 5708, 21 November 1979 as 1 Muharram 1400, and 11 March 2024 as 1 Ramadan 1445, before any visitor used it.
Worked examples
1 January 2000, Gregorian. Julian Day Number 2451545, a Saturday. Julian calendar: 19 December 1999. Hebrew: 23 Tevet 5760. Islamic tabular: 24 Ramadan 1420. One day, four names, and the day number under all of them, which is why astronomers keep their records in it.
The October Revolution, entered as recorded. Russia recorded the revolution on 25 October 1917, Julian. Converted: 7 November 1917 Gregorian, a Wednesday, which is why the October Revolution’s anniversary was celebrated in November, the calendar reform having moved the date and kept the name.
Ramadan’s first day, 1445. Entered in the Islamic calendar as 1 Ramadan 1445, the conversion gives 11 March 2024 Gregorian, a Monday (tabular; the observed start followed local announcements, as the disclaimer notes). Entered the other way, the same Gregorian date returns the same Islamic one, round-trip exactness being the minimum a converter owes.
Reading your results
Read the row of the calendar you need, and use the day number to cross-check anywhere: most reference works and ephemerides can start from it. For family and historical research the typical pattern is Julian-in, Gregorian-out for pre-reform records; for religious planning it is the reverse question (when does a Hebrew or Islamic date fall this year in Gregorian terms), and both directions are the same single calculation here, which is the point of the pivot design.
Limitations
- Islamic: tabular, not observational. The arithmetical calendar is exact as a rule system and usually within a day of observation, but actual months begin on sighting or official announcement, which can shift any given date by one to two days in either direction, differently by country. Umm al-Qura (the Saudi civil calendar) is its own computed variant and is not the table used here.
- Gregorian adoption varied by place and century. The converter converts calendars, not jurisdictions: whether a given town was on Julian or Gregorian time in, say, 1700 is a historical fact about the town, and entering a date in the wrong one of the pair is the commonest conversion error there is.
- Hebrew days begin at sunset. Calendar dates here are civil day dates; a Hebrew date religiously begins the previous evening, which matters for observance timing and not for date conversion as such.
- Very early years are proleptic. Far from the calendars’ home eras the arithmetic still runs, but the results are conventions of extension, increasingly detached from anything a contemporary would have recognised.
Frequently asked questions
Why are Julian and Gregorian dates 13 days apart?
Because the Julian leap rule (every fourth year, no exceptions) overcounts by about three days per four centuries against the solar year, and the Gregorian reform of 1582 both removed the accumulated 10 days and skipped leap days in most century years. The gap has grown by a day at each skipped century leap since: 11 days when Britain switched in 1752, 13 through the 1900s and 2000s, and 14 from March 2100, when 2100 is a Julian leap year and not a Gregorian one. The converter applies the drift for the actual date rather than a fixed offset, so dates on either side of a century boundary convert correctly.
How does the Hebrew calendar keep both months and seasons?
By being lunisolar with a rule: months follow the moon (29 or 30 days) and a leap month, Adar I, is inserted in seven years of each 19-year cycle, which keeps the festivals in their seasons over any long run, at the price of year lengths varying from 353 to 385 days and month lengths (Heshvan, Kislev) varying by year type. The converter implements the full arithmetic, postponements included, which is why its Hebrew conversions match published calendars on the check dates in the method section rather than approximating.
Is the Islamic date here the one used for Ramadan and Eid?
It is the tabular civil date, which most printed converters also show, and it is frequently the same as the observed date, but the religious month’s start is determined by moon sighting and the announcement of the relevant authority, and can differ by a day or two from the table, varying by place. For observance, follow the announced date for your community; for conversion, genealogy and records, the tabular date on this page is the consistent reference, labelled as what it is in the results and the disclaimer.
What is the Julian Day Number used for?
It is a single continuous count of days (from a remote epoch in 4713 BC) that astronomers, ephemerides and calendar scholars use as the neutral pivot between calendars, exactly as this page uses it internally and prints it in the results. Any two dates’ day numbers subtract to the exact number of days between them under the rules, with no months or leap years in the way, which is why it catches conversion errors that date-to-date comparisons miss.
Related calculators
Method: conversion through the Julian Day Number using the documented algorithms named on this page; Hebrew calendar with full postponement rules; Islamic calendar in its arithmetical (tabular civil) form with the observational caveat stated. Last reviewed: 14 October 2026.