Leap year This makes it easy to convert dates to or from Gregorian. The Thai solar calendar uses the Buddhist Era (BE), but has been synchronized with the Gregorian since AD 1941. From AD 8 the Julian calendar received an extra day added to February in years that are multiples of 4. The Coptic calendar and Ethiopian calendar also add an extra day to the end of the year once every four years before a Julian 29-day February. This rule gives an average year length of 365.25 days. However, it is 11 minutes longer than a tropical year. This means that the
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.