Leap year

Both the Gregorian calendar (solar calendar) and the Lunar calendar (lunisolar calendar) have leap years.

The Gregorian calendar is based on the time it takes for the Earth to orbit the Sun from one vernal equinox to the next (approximately 365.2422 days). By calculating each year as 365 days, an extra day is added every four years in a leap year, where February has 29 days instead of the usual 28 days in a non-leap year.

The Lunar calendar, on the other hand, is based on the time it takes for the Moon to orbit the Earth (approximately 29.5306 days) for one lunar month. As a result, a regular lunar year only consists of 354 or 355 days. Due to the significant difference from 365.2422 days, it is necessary to add an extra month (29-30 days) in a leap year, known as a leap month. There is a pattern of "leap year every three years, with an additional leap year every five years, and seven leap years in a 19-year cycle."