The Coptic year is the extension of the ancient Egyptian civil year, retaining its subdivision into the three seasons, four months each. The three seasons are commemorated by special prayers in the Coptic Liturgy. This calendar is still in use all over Egypt by farmers to keep track of the various agricultural seasons. The Coptic calendar has 13 months, 12 of 30 days each and one at the end of the year of 5 days in length, except in leap years when the month is 6 days. Today, and until 2099, the year starts on 11 September in the Gregorian Calendar or on the 12th in the year before (Julian) Leap Years. The Coptic Leap Year follows the same rules as the Julian Calendar so that the extra month always has six days in the year before a Julian Leap Year.
The Feast of Neyrouz marks the first day of the Coptic year. Ignorant of the Persian language for the most part, the Arabs confused the Egyptian new year's celebrations, which the Egyptians called the feast of Ni-Yarouou (the feast of the rivers), with the Persian feast of Nowruz. The misnomer remains today, and the celebrations of the Egyptian new year on the first day of the month of Thout are known as the Neyrouz. Its celebration falls on the 1st day of the month of Thout, the first month of the Egyptian year, which for 1901 to 2098 usually coincides with 11 September, except before a Gregorian leap year when it is 12 September. Coptic years are counted from 284 AD, the year Diocletian became Roman Emperor, whose reign was marked by tortures and mass executions of Christians, especially in Egypt. Hence, the Coptic year is identified by the abbreviation A.M. (for Anno Martyrum or "Year of the Martyrs"). The first day of the year I of the Coptic era was 29 August 284 in the Julian calendar. Note that A.M. abbreviation is also used for unrelated calendar eras (such as the Byzantine and Jewish calendar epochs) which start at the putative creation of the world; it then stands for Anno Mundi.
Every fourth Coptic year is a leap year without exception, as in the Julian calendar, so the above-mentioned new year dates apply only between 1900 and 2099 inclusive in the Gregorian Calendar. In the Julian Calendar, the new year is always 29 August, except before a Julian leap year when it is 30 August. Easter is reckoned by the Julian Calendar in the Old Calendarist way.
To obtain the Coptic year number, subtract from the Julian year number either 283 (before the Julian new year) or 284 (after it).