Editing Intercalation (timekeeping) (section)

== Solar calendars ==
{{further|Intercalary month (Egypt)}}
The solar or [[tropical year]] does not have a whole number of days (it is about 365.24 days), but a [[calendar year]] must have a whole number of days. The most common way to reconcile the two is to vary the number of days in the calendar year.

In solar calendars, this is done by adding an extra day ("leap day" or "intercalary day") to a [[common year]] of 365 days, about once every four years, creating a [[leap year]] that has 366 days ([[Julian calendar|Julian]], [[Gregorian calendar|Gregorian]] and [[Indian national calendar]]s).

The [[Decree of Canopus]], issued by the pharaoh [[Ptolemy III Euergetes]] of [[Ancient Egypt]] in 239 BC, decreed a solar leap day system; an Egyptian leap year was not adopted until 25 BC, when the Roman Emperor [[Augustus]] instituted a reformed [[Coptic calendar|Alexandrian calendar]].

In the [[Julian calendar]], as well as in the [[Gregorian calendar]], which improved upon it, intercalation is done by adding an extra day to February in each leap year. In the Julian calendar this was done every four years. In the Gregorian, years divisible by 100 but not 400 were exempted in order to improve accuracy. Thus, 2000 was a leap year; 1700, 1800, and 1900 were not.

Epagomenal<ref>From ἐπαγόμενος, ''epagomenos'' (present participle passive of ἐπάγειν, ''epagein'' "to bring in") + -al</ref> days are days within a solar calendar that are outside any regular month. Usually five epagomenal days are included within every year ([[Egyptian calendar|Egyptian]], [[Coptic calendar|Coptic]], [[Ethiopian calendar|Ethiopian]], Mayan [[Haab']] and [[French Republican Calendar]]s), but a sixth epagomenal day is intercalated every four years in some (Coptic, Ethiopian and French Republican calendars).

The [[Solar Hijri calendar]], used in Iran, is based on solar calculations and is similar to the Gregorian calendar in its structure, and hence the intercalation, with the exception that its [[epoch]] is the [[Hijrah]].<ref>{{cite web | url=http://www.alislam.org/library/history/ahmadiyya/73.html | title=Hijri-Shamsi Calendar | publisher=Al Islam | access-date=June 14, 2014}}</ref>

The [[Bahá'í calendar]] includes enough epagomenal days (usually 4 or 5) before the last month ({{lang|ar|علاء}}, ''ʿalāʾ'') to ensure that the following year starts on the [[March equinox]]. These are known as the [[Ayyám-i-Há]].