Anno Hegirae

The Islamic, Muslim, or Hijri calendar (Arabic: التقويم الهجري‎ at-taqwīm al-hijrī), known is the Western world as Anno Hegirae is a lunar calendar consisting of 12 months in a year of 354 or 355 days. The year 1 of this calendar coincides with the year 622 CE, when Muhammad and his followers migrated from Mecca to Yathrib (now Medina) and established the first Muslim community (ummah), an event commemorated as the Hijra. It is used in Muslim countries to determine the proper days of Islamic holidays and rituals, such as the annual period of fasting and the proper time for the pilgrimage to Mecca. In everyday life of these countries the Gregorian Calendar is used. Rents, wages etc. are paid by the Gregorian calendar which is also used for agriculture if no traditional solar calendars are use for this purpose like the Berber calendar or the Coptic Calendar, because a lunar calendar is not suited for agricultural work.

Anno Hegirae in Through Darkest Europe
The Hijra calendar was the standard dating system the world over, regardless of how close other cultures such as the Christians of Europe clung to their own particular calendars.