Complex sentence: Umar is also remembered for establishing the Islamic calendar; it is lunar like the Arabian calendar, but the origin is set in 622, the year of the Hijra when Muhammad emigrated to Medina.
Split into simpler sentences: Umar is also remembered for establishing the Islamic calendar. It is lunar like the Arabian calendar, but year-zero is set in 622, the year of the Hijra when Muhammad emigrated to Medina.