How it began
The calendar counts its years from the Hijrah, the migration of Muhammad and his followers in 622 CE from Mecca to Yathrib, later called Medina, both in present-day Saudi Arabia. The era itself was introduced later by Umar I, the second caliph. Britannica gives the year as 639 and other accounts as about 638, or 17 years after the Hijrah. Umar set the first day of year one at the first day of the month of Muharram, which corresponds to July 16, 622, in the Julian calendar.
Before it existed
Before the reform, letters carried no year. One account relates that Abu Musa al-Ashari, an official of Umar at Basra, complained that the correspondence he received from the caliph bore no year, so he could not tell which instructions were the most recent. Several conflicting ways of dating were in use at the time, and Umar consulted his companions before settling on the Hijrah as the starting point.
How it works
The year has twelve months, each beginning approximately at the new moon. The months alternate between 29 and 30 days, and the last month, Dhu al-Hijjah, varies in length over a 30-year cycle to keep in step with the moon. A year therefore has 354 or 355 days. Because nothing is added to match the solar year, the months do not stay in the same seasons but move through the solar year.
Where it stands today
Most countries of the Islamic world use the Gregorian calendar for civil matters and the Hijri calendar for religious purposes. It fixes observances such as the fasting month of Ramadan. Dates are marked AH, from the Latin anno Hegirae, in the year of the Hijrah.
