The Unique Ethiopian Calendar
"The Ethiopian calendar is unique in that it belongs neither to the Julian nor to the Gregorian calendar systems."
Historical literature shows that calendars across the world are based on the work of ancient Egyptian astronomers who calculated that the solar year consists of 365 days. However, the exact calendar division systems diverged.
Months in the Ethiopian calendar are based on the book Metsehafe-Hissab (Book of Calculation). It consists of 12 months of exactly 30 days each, plus a 13th month of 5 days (6 days during a leap year), known as Pagume. This is where the slogan "13 Months of Sunshine" originates.
Because of a different calculation regarding the birth date of Jesus Christ, the Ethiopian calendar is 7 to 8 years behind the Western calendar, celebrating its New Year (Enkutatash) on September 11th.