Editing Epact (section)

===Solar and lunar years===

A [[tropical year|solar calendar year]] has 365 days (366 days in [[leap year]]s).  A [[lunar year|lunar calendar year]] has 12&nbsp;lunar months which alternate between 30 and 29&nbsp;days for a total of 354&nbsp;days (in leap years, one of the lunar months has a day added; since a lunar year lasts a little over {{sfrac|354|1|3}}&nbsp;days, a leap year arises every second or third year rather than every fourth.)

If a solar and lunar year start on the same day, then after one year the start of the solar year is 11&nbsp;days after the start of the lunar year. These excess days are epacts, and have to be added to the lunar year to complete the solar year; or from the complementary perspective they are added to the day of the solar year to determine the day in the lunar year.

After two years the difference is 22&nbsp;days, and after 3&nbsp;years, 33&nbsp;days. Whenever the epact reaches or exceeds 30&nbsp;days, an extra (embolismic or [[Intercalation (timekeeping)#Lunisolar calendars|intercalary]]) lunar month is inserted into the lunar calendar, and the epact is reduced by 30&nbsp;days.

Leap days extend both the solar and lunar year, so they do not affect epact calculations for any other dates.{{explain|date=May 2023|reason=Solar leap days occur in different years than lunar leap days; how could they not (erratically) throw the calculations one day off?}}