The Ethiopian calendar (Amharic: ዐውደ ወር; Ge'ez: ዐውደ ወርኅ; Tigrinya: ዓዉደ ኣዋርሕ), or Geʽez calendar (Geʽez: ዐውደ ወርኅ; Tigrinya: ዓዉደ ኣዋርሕ, Amharic: የኢትዮጲያ ዘመን አቆጣጠር) is the official state civil calendar of Ethiopia and serves as an unofficial customary cultural calendar in Eritrea, and among Ethiopians and Eritreans in the diaspora. It is also an ecclesiastical calendar for Ethiopian Christians and Eritrean Christians belonging to the Orthodox Tewahedo Churches (Ethiopian Orthodox Tewahedo Church and Eritrean Orthodox Tewahedo Church), Eastern Catholic Churches (Eritrean Catholic Church and Ethiopian Catholic Church), and Protestant Christian P'ent'ay (Ethiopian-Eritrean Evangelical) Churches. The Ethiopian calendar is a solar calendar that has much in common with the Coptic calendar of the Coptic Orthodox Church of Alexandria and Coptic Catholic Church, but like the Julian calendar, it adds a leap day every four years without exception, and begins the year on 11 or 12th of September in the Gregorian calendar (from 1900 to 2099). Its epoch (first day of first year) equates to 29 August 8 AD. The neighbouring Coptic calendar is very similar to the Ethiopian calendar, except that it has a different epoch (29 August, 284 AD) and different names for the days of the week and months of the year.

The Ethiopian calendar has twelve months, all thirty days long, and five or six epagomenal days, which form a thirteenth month. The Ethiopian months begin on the same days as those of the Coptic calendar, but their names are in Geʽez. A sixth epagomenal day is added every four years, without exception, on 29 August of the Julian calendar, six months before the corresponding Julian leap day. Thus, the first day of the Ethiopian calendar year, 1 Mäskäräm, for years between 1900 and 2099 (inclusive), is usually 11 September (Gregorian). It falls on 12 September in years before the Gregorian leap year, however.

The Ethiopian New Year is called Kudus Yohannes in Geʽez and Tigrinya, while in Amharic, the official language of Ethiopia, it is called Enkutatash meaning "gift of jewels". It occurs on 11 September in the Gregorian calendar; except for the year preceding a leap year, when it occurs on 12 September. The Ethiopian Calendar Year 1998 Amätä Məhrät ("Year of Mercy") began on Gregorian calendar 11 September 2005. The Ethiopian calendar years 1992 and 1996, however, began on the Gregorian dates of 12 September in 1999 and 2003 respectively.^{[citation needed]}

This date correspondence applies for Gregorian years 1900 to 2099. The Ethiopian calendar leap year is every four without exception, while Gregorian centurial years are only leap years when exactly divisible by 400; thus, a set of corresponding dates will most often apply for a single century. As the Gregorian year 2000 is a leap year, the current correspondence lasts two centuries instead.^{[citation needed]}

The start of the year (the Feast of El-Nayrouz) falls on 29 or 30 August (on the year just before the Julian leap year). This date corresponds to the Old-Style Julian calendar; the start of the year has been transferred forward in the currently used Gregorian calendar to 11 or 12 September (on the year just before the Gregorian leap year). This deviation between the Julian and the Gregorian Calendar will increase with the passing of the time.

To indicate the year, followers of the Ethiopian and Eritrean churches today use the Incarnation Era, which dates from the Annunciation of the Incarnation of Jesus on 25 March AD 9 (Julian), as calculated by Annianus of Alexandria c. 400; thus, its first civil year began seven months earlier on 29 August AD 8. Meanwhile, Europeans eventually adopted the calculations made by Dionysius Exiguus in AD 525 instead, which placed the Annunciation nine years earlier (Anno Domini#New year) than had Annianus. This causes the Ethiopian year number to be eight years less than the Gregorian year number from January 1 until 10 or 11 September, then seven years less for the remainder of the Gregorian year.

In the past, a number of other eras for numbering years were also widely used in Ethiopia, Eritrea, and the Kingdom of Aksum.

The most important era – once widely used by Eastern Christianity, and still used by the Coptic Orthodox Church of Egypt – was the Era of Martyrs, also known as the Diocletian Era, or the era of Diocletian and the Martyrs, whose first year began on 29 October 328.