The Hegira marks the first year of Islam, July 16, 622. Islamic years have a 12 month period, based on the phases of the moon, which is much like ours. A new month starts when the crescent moon is first seen. After ten years of struggling Muhammad (pbuh) and his followers fled to Medina, that migration is known as the Hegira. At first the people of Medina were hostile towards him, they probably felt threatened by his knew idea of religion but soon accepted him as a prophet. Muhammad (pbuh) based his laws on his messages from God he recieved and called his religion Islam, which means "submission" to God.