The Ethiopian calendar or Ethiopic calendar is the principle calendar used in Ethiopia. It is based on the old Alexandrian calendar , which is the even older Egyptian calendar with a leap day added every four years, similar to the Julian calendar. Like the Alexandrian and Coptic calendars, the Ethiopian calendar has twelve months of 30 days each plus five or six epagomenal days (usually called a thirteenth month), but with Amharic names. The sixth epagomenal day is added every four years on 29 August in the Julian calendar, six months before the Julian leap day. Thus the first day of the Ethiopian year, 1 Maskaram, is usually 29 August (Julian), but is 30 August (Julian) six months before the Julian leap day. For Gregorian years between 1901 and 2099 inclusive, the equivalent Gregorian dates are 11 September and 12 September. The Ethiopian calendar uses the Incarnation Era, which dates the Incarnation of Jesus to 25 March 9 (Julian) (chosen by Annianus of Alexandria at the beginning of the fifth century), thus its first civil year began seven months earlier on 29 August 8 (Julian). This causes the Ethiopian year number to be eight years less than the Gregorian year number from 1 January until 10 or 11 September, then seven years less for the remainder of the Gregorian year.