唔係所有嘅曆法系統都會用太陽年做單位。太陰曆就係用月相變化嚟計算日期嘅曆法。因為回歸年嘅長度並唔係月相週期嘅整數倍，單純嘅太陰曆好快就會同季節對齊唔到。不過同其他自然現象會對齊得好好，例如潮汐。伊斯蘭曆就是太陰曆嘅一種。
Not all calendar systems will use solar years as units. The lunar calendar is a calendar that uses the changes in the phases of the moon to calculate dates. Because the length of the return year is not an integer multiple of the lunar cycle, a simple lunar calendar will soon become out of line with the seasons. However, it aligns well with other natural phenomena, such as tides. The Islamic calendar is a type of lunar calendar.