Answer:
The Islamic calendar is reckoned from the time of migration (Hijra) of Prophet Mohammad (Peace Be Upon Him) from Mecca to Madina. The Prophet's decision to migrate from Mecca came after several years of inhuman treatment of the faithful by the powerful tribes who were united despite all their feuds to stop the spread of Islam.
The migration of the Prophet and his followers took place in the thirteenth year of the mission. It all started after the people of Medina offered the Prophet to migrate to seek refuge in their land and guaranteed him protection amid growing hostility against his message in Medina.
The delegation promised itself and its fellow inhabitants to accept Hazrat Muhammad (PBUH) in their society and physically protect him as one of them. After that, in 622 AD, the Holy Prophet (PBUH) ordered his supporters to migrate from Mecca to Medina.