Answer:
The Islamic calendar is lunar, which means it's based on the phases of the moon. It is either 354 or 355 days long, and began in the year 622 CE when the Prophet Muhammad migrated from Mecca to Medina.
The Gregorian calendar is solar. It is about 365 days long, and numbers 0 CE at the birth of Jesus. It is a reform of the Julian calendar as used up to Pope Gregory's time, together with a reform of the lunar cycle used by the Church along with the Julian calendar for calculating dates of Easter.