Ethiopic scholar Ephraim Isaac explains the calendar here:
The Ethiopians are the only nation using their own calendar as the calendars of the rest of the world are based on the work of the old Egyptian astronomers who discovered, as early as three to four thousand years BC - that the solar or sidereal year lasted slightly less than 365-and-a-quarter days. It was however left to the astronomers of the Alexandrian school to incorporate this knowledge into some sort of calendar, and it was these astronomers who also came up with the idea of leap years. Consequently, the Romans, under Julius Caesar, borrowed their reformed calendar from the Alexandrian science and adopted it for the Western world.Happy New Year to the Ethiopians too!
The Copts handed this calendar, together with their method of computing the date of Easter, on to their descendant Church in Ethiopia. The Ethiopian year therefore has something in common with the Western year, having been derived from the same source. The Ethiopians, as such, retain the old Egyptian system whereby the year was divided into 12 months of 30 days, with one additional month of five days (six days during the leap year). The Ethiopian calendar therefore falls seven to eight years behind the Western model and has done so since early Christian times.
Ethiopian Millennium activist, Professor Ephraim Isaac, explains that the Ethiopian calendar is not the same as the Julian or the Gregorian calendars used widely by the rest of the world.
"Some people consider the Ethiopian calendar as being the same as the Julian calendar. But ours is not exactly Julian. Originally, the Julian calendar was a calendar from about 40 BC. There were 365 days during the leap year. Both Julian and Gregorian calendars are 30 or 31 days with February being either 28 or 29 days. Now the Gregorian calendar is the revision of the Julian calendar, which Pope Gregory edited or decided to change according to certain calculation.
They share certain characteristics in terms of 365 and 366 days during the leap year. But our calendar has 30 days every month and additional five or six days during the leap year. Ethiopian calculation is documented in a book called Mesafe Hisab, which is found in Ethiopia.
It is based on the early Christian calendar, which is derived from the Alexandrian Jewish calendar. What is significant is that both Western and Ethiopian calendars both calculate the era (days) from the day Jesus Christ, the founder of Christianity was born.