When did Ethiopians Start using calendar system ?

Ethiopians have been using their own calendar system, known as the Ethiopian calendar or the Ge'ez calendar, since ancient times. The exact date of its origin is not known, but it is believed to have been in use since at least the 4th century AD. The Ethiopian calendar is based on the ancient Egyptian calendar and has 13 months, with 12 months of 30 days each and a 13th month of 5 or 6 days, depending on whether it is a leap year or not.