The lunar based year has only 354 days, which is 11 days short of the solar year of 365 days on which the seasons depend. This means the lunar calendar moves ahead of the seasons by 11 days each year.

To solve this problem, an extra month has to be inserted every 2 to 3 years. We call this calendar lunisolar because it is a lunar calendar that keeps in line with the solar seasons by the intermittent addition of these extra months.

Adding extra months keeps the calendar in line with the solar seasons, but the true length of a lunation is 29.53 days, not 29.5, and the true length of an astronomical lunar year is 354.367 days, not 354. This means that every few years an extra day has also to be added at the end of the year to keep in line with the true astronomical lunar year.