Which country has 13 months in a year?

Ethiopia uses a unique calendar system with 13 months in a year - 12 months of 30 days each, plus a 13th month called 'Pagume' with 5 or 6 days depending on whether it's a leap year. This gives Ethiopia a calendar that is about 7-8 years behind the Gregorian calendar used in most of the world. The Ethiopian calendar is based on the Alexandrian calendar, which itself derives from the Egyptian calendar.