The Islamic calendar
The Hijri calendar is a lunar calendar of twelve months, counted from the Prophet's migration (hijra) from Makkah to Madinah in 622 CE. Allah says: 'Indeed, the number of months with Allah is twelve' (Surah at-Tawbah 9:36). A lunar year is about 354 days, eleven days shorter than the solar year, so Islamic dates move earlier through the seasons each year.
Why dates can differ by a day
This tool uses the Umm al-Qura calculated calendar, used officially in Saudi Arabia and by most apps. But the start of each month is traditionally confirmed by sighting the new moon, which can fall a day before or after the calculation depending on your location. For Ramadan, the two Eids, and Hajj, follow your local moon-sighting authority.
The twelve months
The months are Muharram, Safar, Rabi al-Awwal, Rabi al-Thani, Jumada al-Ula, Jumada al-Thani, Rajab, Shaban, Ramadan, Shawwal, Dhul-Qadah, and Dhul-Hijjah. Four are sacred - Dhul-Qadah, Dhul-Hijjah, Muharram, and Rajab - in which fighting was traditionally forbidden.
How the conversion works
This converter uses the Umm al-Qura calendar - the calculated Islamic calendar published in Saudi Arabia and built into modern browsers - anchored to the Hijra in 622 CE. Conversion runs instantly in both directions, entirely on your device. The one honest caveat: the Islamic months used for worship begin with the actual sighting of the new crescent, and a sighted month can start a day before or after the calculated one. That is why the result carries a plus-or-minus-one-day note - for documents, records, and planning the calculated date is exactly what you want; for the start of Ramadan or Eid, follow your local sighting authority.
Everyday uses
Knowing a date in both calendars turns out to be quietly useful all year. Convert your birth date to know your age in hijri years - which can matter for religious obligations that begin at a certain age. Set your zakat anniversary: zakat is due after a full lunar year, so the hawl date you record should be a hijri one, and this converter keeps it stable against the Gregorian drift of about eleven days per year. Planners use it for the white days (the 13th, 14th, and 15th of each lunar month, when fasting is recommended), for knowing how far away the sacred months are, and for reading historical Islamic dates in their own calendar.