9:36 The count of months, with God, is twelve months in God’s book, the day He created the heavens and earth. Four of these Months are Sacred, forbidden for all warfare. This is the Firm Religion, the right way. So, do not wrong yourselves by fighting during the Sacred Months. However, you may fight the idolaters in any or all these months if they fight against you in any or all of them (2:193-195). And know that God is with those who restrain themselves.
The Qur'an speaks about 12 months so there must have been some monthly, yearly system in place in the beginning, before Umar apparently did something.