因為回歸年嘅長度唔係一日嘅整數倍，因此太陽曆有啲年嘅日數會同其他年嘅天數唔一様，例如閏年要加一日（閏日）。陰曆嘅月或者陰陽曆入面一年嘅月份數，都會有類似嘅情形，叫做置閏。大多數太陽曆嘅一年都冇辦法分為長度相同，唔變動嘅十二個月。
Because the length of the return year is not an integer multiple of a day, the solar calendar has some years in which the number of days is different from the number of days in other years, such as leap years with a day (leap day). The number of months in the lunar calendar or the number of months in a year in the lunisolar calendar will also have a similar situation, which is called intercalation. Most solar calendar years cannot be divided into 12 months of the same length and unchanged.