历法是用年、月、日等时间单位计算时间的方法。

主要分为阳历、阴历和阴阳历三种。阳历亦即太阳历，其历年为一个回归年，现时国际通用的公历（西历）即为太阳历的一种，亦简称为阳历；阴历亦称月亮历，或称太阴历，其历月是一个朔望月，历年为12个朔望月，其大月30天，**29天，伊斯兰历即为阴历的一种；阴阳历的平均历年为一个回归年，历月为朔望月，因为12个朔望月与回归年相差太大，所以阴阳历中设置闰月，所以这种历法与月相相符，也与地球绕太阳周期运动相符合。中国的农历就是阴阳历的一种。

历法中包含的其他时间元素（单位）尚有：

A calendar is a system of organizing days for social, religious, commercial or administrative purposes. This is done by giving names to periods of time, typically days, weeks, months, and years. A date is the designation of a single, specific day within such a system. A calendar is also a physical record (often paper) of such a system. A calendar can also mean a list of planned events, such as a court calendar or a partly or fully chronological list of documents, such as a calendar of wills.