Hijrah

Hijra (هِجْرَة), or withdrawal, is the word that is used for the movement of Muhammad and most of his followers from Mecca to Medina in 622. The Hijra marks the beginning of the Islamic calendar. Alternate spellings of this Arabic word in the Latin alphabet are Hijrah, or Hegira in Latin.
Chronology of the Hijra.
The Muslim dates are in the Islamic calendar extended back in time. The Western dates are in the Julian calendar. The Hijra is celebrated annually on 8 Rabi' I, about 66 days after 1 Muharram, the first day of the Muslim year. Many writers confuse the first day of the year of the Hijra with the Hijra itself, erroneously stating that the Hijra occurred on 1 Muharram AH 1 or 16 July 622.