Why Islamic dates shift every year, how the lunar calendar works, and how to convert dates accurately.
Published September 1, 2026 · Reviewed by the Ihsabha Editorial Team
Anyone who has tried to figure out when Ramadan or Eid falls on the Gregorian calendar has run into the same puzzle: the date keeps moving. Understanding why makes converting between the Hijri and Gregorian calendars — for birthdays, contracts, zakat anniversaries, or religious occasions — much easier.
The Gregorian calendar is solar, built around the Earth's orbit of the sun (365 or 366 days). The Hijri calendar is purely lunar, made up of 12 months that each follow a full cycle of the moon. A lunar year totals only 354 or 355 days — about 10 to 11 days shorter than a solar year, with no extra month added to compensate.
Because the Hijri year is consistently shorter, any fixed Hijri date — like the 1st of Ramadan — falls about 10 to 11 days earlier on the Gregorian calendar than it did the previous year. Over roughly 33 Gregorian years, a Hijri date will have cycled all the way through every Gregorian season and returned close to where it started.
For everyday planning, the calculated conversion is accurate and widely used; for religious observances tied to specific months like Ramadan or Dhu al-Hijjah, local moon-sighting announcements remain the final word.
Someone born on 15 Ramadan 1400 AH wants to know their Gregorian birthday. Converting month by month using the calculated Hijri calendar places that date at 28 July 1980 — a conversion that would take significant manual effort to work out by hand across 1,400 years of lunar months, but is instant with a proper converter.
Convert any date instantly, both directions. Ihsabha's Hijri-Gregorian Converter handles the full calculation for you, whether you're converting a birthday, a contract date, or checking your zakat hawl anniversary.
The Hijri calendar is purely lunar, with a year of about 354 or 355 days — roughly 10 to 11 days shorter than the Gregorian solar year. Because of this gap, Hijri dates drift earlier relative to the Gregorian calendar each year and cycle through all four seasons over about 33 Gregorian years.
A Hijri year has 12 lunar months and is either 354 or 355 days long, depending on the length of each month, which alternates based on the moon's cycle.
Calculated conversions are highly accurate for planning purposes, but the start of some months can shift by a day depending on local moon-sighting decisions, so official religious dates are sometimes confirmed separately by local authorities.
Calculated conversions are excellent for general planning, but for religious dates that depend on the sighting of a new moon, always confirm the final date with your local mosque or religious authority closer to the time.