Calendar is mentioned in the Quran in these verses:

[9:36] The count of months, as far as God is concerned, is twelve. This has been God’s law, since the day He created the heavens and the earth. Four of them are sacred. This is the perfect religion; you shall not wrong your souls (by fighting) during the Sacred Months. However, you may declare all-out war against the idol worshipers (even during the Sacred Months), when they declare all-out war against you, and know that God is on the side of the righteous.

[9:37] Altering the Sacred Months is a sign of excessive disbelief; it augments the straying of those who have disbelieved. They alternate the Sacred Months and the regular months, while preserving the number of months consecrated by God. They thus violate what God has consecrated. Their evil works are adorned in their eyes. God does not guide the disbelieving people.

To learn more, read the Quran in English.

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