“He is Who gave the Sun radiance and Moon the light, and determind the stages (for waxing and waning of Moon) that you may learn the calculation of years and reckoning of time. Allah has created all this with a rightful purpose (rather than out of play). He expounds His signs for the people who know,” – Holy Qur’an, Surah Younus, Verse 5.
Islamic Calendar, also known as Hijri Calendar (Latin ‘anno hegirae’ or AH), was established in 638 CE by the second Caliph of Islamic State, Hazrat Umar bin Khattab (ra) – in order to have a common Calendar within different countries under Islamic State, such as Misr (Egypt), Faris (Persia), and al-Sham (Syria)” – which all had their ancient non-Islamic Calendars based on solar phases with 365 days – but each had different starting point (epoch).
Pre-Islamic Persian Calendar used June 16, 632 CE – the date of the accession of the last Sasanid ruler, King Tazdagird III.
Pre-Islamic Syria, being part of Byzantine Empire, used a form of the Roman (Julian) Calendar, which begins of October 1, 312 BC.
Pre-Islamic Egypt used Coptic Calendar with starting point of August 29, 284 CE.
Hazrat Umar (ra) chose the day of the Prophet Muhammad’s (pbuh) migration (Hijra) from his birth place, Makkah, to Yasrab (Madinah), where he was to be buried after his death. This even took place on Muharram 1, 1 AH (July 16, 622 CE).
Since Islamic Calendar’s lunar year has 354 days, eleven days less than the Gregorian solar year – it’s therefore, not synchronized to the seasons. Its festivals, which fall on the same days of the same lunar months each year, make the round of the seasons every 33 solar years.
An approximate conversion of dates from Islamic to Gregorian Calendar and vice versa can be achieved by using the converter here.