October 27, 2026 · Everyday Math · 5 min read

Two Calendars, One Date: How Hijri–Gregorian Conversion Works

The Gregorian calendar tracks the sun; the Hijri (Islamic) calendar tracks the moon. Both count days faithfully, but their years disagree — which is why converting between them is genuinely a calculation, not just a relabeling.

Why the calendars drift apart

A lunar month — one new moon to the next — averages about 29.53 days, so twelve lunar months come to roughly 354 days. The solar year is about 365.24 days. The gap of about 11 days per year means every Hijri date moves about 11 days earlier in the Gregorian calendar each year, completing a full cycle of the seasons roughly every 33 Gregorian years. Ramadan, fixed in the lunar calendar, therefore rotates through spring, winter, autumn and summer over a lifetime — while a Gregorian anniversary like December 25 stays in the same season forever.

Calculated calendars vs. moon sighting

Because the new moon isn't visible everywhere at the same time, Muslim communities have historically announced month starts after local sighting. For civil purposes, though, countries need dates decided in advance. The Umm al-Qura calendar — the official civil calendar of Saudi Arabia and the most widely referenced tabular Hijri calendar — precomputes month lengths so planners, airlines and software can agree on dates years ahead. A converter built on Umm al-Qura, like the one in your browser's internationalization system, may differ from a local moon-sighting announcement by a day; that's expected, not an error.

Month lengths and the missing day 30

Hijri months have 29 or 30 days, depending on when the following new moon falls. When you enter day 30 for a month that Umm al-Qura assigns only 29 days, the date simply doesn't exist — a good converter rejects it and suggests day 29 rather than silently shifting. The twelve months run Muharram, Safar, Rabiʻ al-awwal, Rabiʻ al-thani, Jumada al-ula, Jumada al-akhirah, Rajab, Shaʻban, Ramadan, Shawwal, Dhu al-Qiʻdah and Dhu al-Hijjah.

What conversion is used for

• Religious planning: finding the Gregorian dates of Ramadan, Eid al-Fitr and Eid al-Adha for the years ahead.
• Official paperwork: documents issued in Gulf countries often carry Hijri dates that need a Gregorian equivalent for visas, contracts and insurance forms.
• Anniversaries: checking Islamic commemorations against your everyday calendar.
• Curiosity: knowing today's date in both systems at a glance.

One more wrinkle worth knowing: reliable tabular conversion covers roughly Hijri years 1300–1600 — Gregorian years from the late 1800s to the late 2100s. Outside that window, honest tools warn you instead of inventing an answer.

Counting time the other way? Age Math: How Old Are You in Days, Hours and Minutes? explores exact age from a date of birth.