If a large part of the world has adopted the Gregorian calendar, consisting of 365 days a year and 366 days in a leap year, the Ethiopian calendar is built on the basis of the Coptic calendar, of which it is very different.
It contains 13 months including 12 months with 30 days each, and a 13th month at the end of the year called the infill month, which lasts 5 days and 6 days in a leap year. In addition, like the Julian calendar (used in ancient Rome), its year begins in September and an extra day is added every 4 years.
The Gregorian and Coptic calendars are not the only calendars. There are other types: the Chinese calendar, the Hebrew calendar, the Balinese pawukon, the Muslim calendar, the Persian calendar.