Sri Lanka Hijri Date Converter (Islamic Calendar)

The Islamic (Hijri) calendar is purely lunar: 12 months of 29 or 30 days totalling about 354 days, so each Hijri year starts roughly 11 days earlier in the Gregorian calendar than the last. Converting between the two calendars can't be done by simple subtraction — the tool routes every date through the Julian Day Number (JDN), a continuous count of days that belongs to no calendar, then out into the target calendar.

A Gregorian date becomes a JDN with the Fliegel–Van Flandern formula, and a JDN becomes a Hijri date with the tabular (“islamic-civil”) algorithm from Reingold & Dershowitz's Calendrical Calculations — the same scheme behind the islamic-civil calendar in ICU and most programming platforms. The epoch is anchored so that 1 Muharram 1 AH equals JDN 1948440, i.e. 19 July 622 CE in the proleptic Gregorian calendar. Reversing the same formulas turns a Hijri date back into a Gregorian one, and the weekday comes straight from the JDN: weekday = (JDN + 1) mod 7.

The tabular calendar fixes month lengths by a 30-year rule: odd months have 30 days, even months 29, and the twelfth month (Dhu al-Hijjah) gains a 30th day in the 11 leap years of each 30-year cycle. Because it is a fixed rule, it is fully deterministic and exactly reversible — this tool's engine was checked against every single day from 700 CE to 2100 CE and each one converts to Hijri and back to the same date.

What arithmetic cannot capture is the actual sighting of the new crescent moon, which is how the month truly begins in Islamic practice. In Sri Lanka that call is made by the All Ceylon Jamiyyathul Ulama (ACJU). Depending on visibility, the observed month can start a day before or after the calculation — the well-known ±1-day difference. Where the internationally referenced Umm al-Qura calendar disagrees with the tabular result for Ramadan, Eid, or Muharram, the tool shows the Umm al-Qura date beside it so the gap is visible. The rule of thumb: use the converter for everyday dating, but follow ACJU for the binding start of Ramadan and the two Eids.

• All Ceylon Jamiyyathul Ulama (ACJU) — official Islamic calendar & moon-sighting announcements for Sri Lanka
• Umm al-Qura calendar (Kingdom of Saudi Arabia) — the internationally referenced calculated Islamic calendar
• Reingold & Dershowitz — Calendrical Calculations (the tabular Islamic calendar and JDN conversions)
• Fliegel & Van Flandern — A Machine Algorithm for Processing Calendar Dates (CACM, 1968)

The conversion engine and its worked examples were last cross-checked on 2026-07-10. Umm al-Qura reference dates and the ACJU caveat are reviewed before each Ramadan and Hajj season.