In the Muslim world, the calendar is based on the cycles of the moon. The Hijri or Islamic lunar calendar consists of twelve months of 29 or 30 days – depending on the moon phase- and is used to pinpoint the days for Islamic holidays. In contrast to the solar calendar, which is used in the west, the months of the lunar calendar change annually by a difference of -10 to -12 days. As a result, each month always falls on a different season and therefore varies from the Gregorian Calendar.

The starting point of Islamic time calculations is the emigration of the Prophet Muhammad from Mecca to Medina in present-day Saudi Arabia in the year 622, known as Hijri, and is when the first Muslim community was founded.