What the Hijri calendar counts from
The Hijri calendar counts its years from the Hijrah, the emigration of the Prophet Muhammad ﷺ and his companions from Makkah to Madinah in 622 CE. The count itself was adopted some years later, during the caliphate of Umar ibn al-Khattab, and year one was set to begin with the Muharram of the year in which the emigration took place rather than with the day of the journey. Years are marked AH, from the Latin anno Hegirae, and in Arabic with the letter هـ.
It is the calendar of Islamic worship. Ramadan, Hajj, the two Eids, the lunar year over which zakat is reckoned and the waiting periods of family law are all counted in Hijri months. That is why a household that runs its working life on the Gregorian calendar still needs to know where it stands in the Hijri one.
A lunar calendar, eleven days short of the sun
Each Hijri month begins with the new crescent moon, the hilal. The Quran describes the new moons as markers of time for people and for the pilgrimage (al-Baqarah 2:189), and the months follow the moon, not the sun. The time from one new moon to the next averages about 29.53 days, so a month has either 29 or 30 days — never 31, and with no fixed pattern you can memorise.
Twelve such months come to 354 or 355 days, about eleven days fewer than the solar year. There is no leap month to close the gap. Every Hijri date therefore arrives about eleven days earlier in the Gregorian calendar each year and passes through every season in a cycle of roughly 33 years. A Ramadan that falls in the long, hot days of summer will fall in short winter days some fifteen years later, and someone who fasts through most of a lifetime will have fasted it in every season.
The twelve months
In order, the months are Muharram, Safar, Rabi' al-Awwal, Rabi' al-Thani, Jumada al-Ula, Jumada al-Akhirah, Rajab, Sha'ban, Ramadan, Shawwal, Dhu al-Qa'dah and Dhu al-Hijjah.
Muharram opens the year, and its tenth day, Ashura, is a day on which many Muslims fast. Ramadan, the ninth month, is the month of obligatory fasting; its last ten nights are when Laylat al-Qadr is sought, and the first of Shawwal, the day after it ends, is Eid al-Fitr. Dhu al-Hijjah, the twelfth, is the month of Hajj: the pilgrims stand at Arafah on the ninth, and the tenth is Eid al-Adha, followed by the days of Tashriq.
Four months are sacred. The Quran states that the number of months with Allah is twelve, of which four are sacred (at-Tawbah 9:36), and they are identified as Dhu al-Qa'dah, Dhu al-Hijjah, Muharram and Rajab. Three of them run together across the turn of the year; Rajab stands on its own. Many communities also mark Rabi' al-Awwal as the month of the Prophet's birth, while others hold no commemoration for it.
Calculated calendars and local moon sighting
The traditional method is sighting. On the evening of the 29th, people look for the new crescent after sunset. If it is seen, the new month begins at that sunset; if it is not — because the moon is too young, or cloud hides it — the current month is completed to 30 days. Many communities, especially in South Asia and in many mosques in Europe and North America, still begin Ramadan and the Eids on a local or regional sighting.
The other method is calculation: the astronomical conditions for each month are worked out ahead of time and the calendar is printed years in advance. This converter uses the Umm al-Qura calendar, the calculated calendar of Saudi Arabia, which is based on astronomical data for Makkah. Saudi Arabia uses it for civil and administrative dates, but announces the start of Ramadan, Shawwal and Dhu al-Hijjah on reports of sighting, so even there the religious date can depart from the printed one. Other countries publish calculated calendars of their own with different criteria; Turkey's Diyanet is one.
The schools of law also differ over whether a sighting in one place binds another. One view holds that a sighting anywhere establishes the month for all Muslims; another holds that regions far enough apart each follow their own horizon. Both positions have long histories, and national authorities follow one or the other.
Why a converted date can be a day out
Put those together and two countries can begin the same month a day apart, occasionally two. The converter is not wrong when that happens; it answers a different question — what the Umm al-Qura calendar says — from the one your local mosque answers.
A second source of mismatch is where the day begins. In Islamic reckoning a day begins at sunset, so each night comes before its day: the first night of Ramadan, when tarawih is first prayed, is the evening before the first fast, and the night of Eid is the evening before the Eid prayer. The converter matches whole Gregorian days to Hijri days, as printed calendars do, so after sunset you are already in the next Hijri day.
Before fixing a fast, an Eid, the day of Arafah, a nikah or the end of a waiting period to a date, check it against the calendar or announcement your own community follows. The announcement for Ramadan and the Eids may come only on the evening before, so for anything planned months ahead, allow for a day either way.
How to read the converter
Choose Gregorian → Hijri to find the Hijri date of a day you know. The result shows the Hijri day, month and year, with the Gregorian date written out underneath, weekday included, so you can confirm you picked the day you meant. It opens on today, taken from your device's clock.
Choose Hijri → Gregorian to go the other way. Enter the day, pick the month from the list — the months are numbered, so Ramadan is 9 and Dhu al-Hijjah 12 — and type the year as a plain number, without AH. The result is the Gregorian date the Umm al-Qura calendar gives. If you ask for the 30th of a month that has only 29 days that year, the converter says the date does not exist rather than handing you a nearby one.
The conversion runs in your browser; nothing you enter is sent anywhere.