It began with the year of the journey of Muhammad and his friends from Mecca to Medina. The Qur’an established a calendar that measured time by the movement of the moon. Regular use of an Islamic calendar began in about 634 CE.
The lunar year is about eleven days shorter than the solar year. Thus, the beginning of the year slips back through the solar system.
The day begins at sunset and ends on the following day at sunset. Each day has twenty-four hours. There is no day of rest as the Jews and Christians have. However, Friday is a special day of remembrance and is used as a day of assembly. The week has seven days.
The month begins two days after the first appearance of a crescent on the new moon. Months alternate with twenty-nine and thirty days. The calendar is based on a thirty-year cycle of 360 lunar months. It is out one day in 2500 years. Common years have 354 days. An intercalary day is added to the twelfth month in eleven of the years of the cycle.