9:36
The number of months in the sight of Allah is twelve (in a year)- so ordained by Him the day He created the heavens and the earth; of them four are sacred: that is the straight usage. So wrong not yourselves therein and fight the Pagans all together as they fight you all together. But know that Allah is with those who restrain themselves.
The passage explains that two primary systems are used worldwide to measure the passage of years: the lunar calendar and the solar calendar. The lunar calendar is based on the cycles of the moon, while the solar calendar is determined by the earth’s movement around the sun. Both systems generally divide the year into twelve months of approximately thirty days, although the solar calendar requires periodic adjustments, such as leap years, to maintain accuracy. Early Muslims were familiar with both systems, and the medieval scholar Ibn al-ʿArabi observed that the solar calendar is useful for regulating worldly affairs, whereas the lunar calendar is particularly suited for religious observances. Consequently, while many Muslims—and similarly Jews—continue to determine religious festivals according to the lunar cycle, they also utilize the solar calendar for practical matters in contemporary society (as cited in Islamic historical commentary).
References:
Emerick, Yahiya. The Meaning of the Holy Qur’an in Today’s English (p. 829). Unknown. Kindle Edition.