Coptic calendar Roman Emperor Augustus imposed the Decree upon Egypt as its official calendar (although it was unsynchronized with the newly introduced Julian calendar which had erroneously been intercalating leap days every third year due to a misinterpration of the leap year rule so as to apply inclusive counting). To distinguish it from the Ancient Egyptian calendar, which remained in use by some astronomers until medieval times, this reformed calendar is known as the Coptic calendar. Its years and months coincide with those of the Ethiopian calendar but have different numbers and names. The Coptic year is the extension of the ancient
Leap year be corrected. A year that is not a leap year is called a common year. For example, in the Gregorian calendar, each leap year has 366 days instead of 365, by extending February to 29 days rather than the common 28. These extra days occur in years which are multiples of four (with the exception of centennial years not divisible by 400). Similarly, in the lunisolar Hebrew calendar, Adar Aleph, a 13th lunar month, is added seven times every 19 years to the twelve lunar months in its common years to keep its calendar year from drifting through the seasons.