Hijri and Gregorian calendars: how to convert a date

The Gregorian and Hijri calendars describe the same passage of time with different rules. The Gregorian calendar is solar. The Hijri calendar used for many Muslim religious dates is lunar, with twelve lunar months and a year of roughly 354 or 355 days.

That difference is why there is no fixed number of days you can simply add or subtract. The Bethemesh Hijri date converter performs the calendar conversion in both directions, while keeping the underlying convention visible.

Why Hijri dates move through the Gregorian seasons

A Gregorian year is about eleven days longer than a Hijri year. Over time, that difference accumulates. Ramadan, Hajj and other dates therefore move earlier through the Gregorian calendar instead of staying in the same season.

This is a feature of the two calendar systems, not an error in conversion.

Calculated calendar versus crescent observation

A digital converter needs a deterministic calendar model. Bethemesh uses the Umm al-Qura calendar support exposed by the browser for Hijri formatting and conversion.

A calculated result is useful for planning, comparing documents and understanding calendar correspondence. It does not mean every community will announce the start of a religious month on that exact civil date. In contexts where the month begins through crescent observation or a religious authority’s decision, a one-day difference can occur.

For a future religious observance, treat a computed conversion as a calendar reference and check the relevant local announcement when that announcement is authoritative for your community.

Why sunset matters

A Gregorian civil date normally changes at midnight. In Islamic religious practice, the next religious day can begin at sunset. The same Gregorian date can therefore map to a different Hijri date in the evening than it did earlier that day.

The converter includes an after sunset option for this reason.

Gregorian to Hijri

To convert a civil date:

1. choose Gregorian → Hijri;
2. enter the Gregorian date;
3. indicate whether the moment is after sunset when relevant;
4. calculate the corresponding Hijri day, month and year.

The output is the result of the selected computational calendar, not a prediction of a future sighting decision.

Hijri to Gregorian

The reverse direction is helpful when an invitation, archive or historical reference gives only a Hijri date. Enter the Hijri day, month and year and calculate the corresponding Gregorian date.

For dates tied to a month whose beginning may still be established by observation, keep the same distinction between a computed correspondence and the date that will ultimately be observed locally.

Date conversion is different from prayer-time calculation

A calendar converter answers which date? Prayer times answer what time? and add another set of variables: geographic coordinates, time zone, solar position and calculation method.

Two cities may share the same Hijri date while having different Fajr, Maghrib or Isha times. That is why Bethemesh keeps date conversion and prayer-time calculation as separate tools.

A practical way to build a dual calendar

If you maintain a personal planner, you can record both systems side by side:

Gregorian date      Calculated Hijri date
17 Sep 2026    →    Hijri correspondence
18 Sep 2026    →    next Hijri correspondence

For observances subject to a local announcement, mark the date as provisional until confirmed. This gives you a useful planning calendar without presenting a calculated value as a universal religious ruling.

Summary

Hijri/Gregorian conversion is reliable once a computational convention is fixed, but calendar calculation and religious observation are not always the same thing. The important questions are which calendar method is being used and whether the time is before or after sunset.

FAQ

Is every Hijri year exactly 354 days?

No. Computational Hijri years can contain 354 or 355 days depending on the calendar system and year.

Why does Ramadan move by about ten or eleven days each year?

Because the lunar Hijri year is shorter than the solar Gregorian year.

Is an online Hijri conversion an official religious date?

Not necessarily. It is the result of a defined calendar method. A community that relies on observation or an official announcement may use a date that differs by a day.