I understand that the religious calendar came into existence either in Abraham’s time or Moses’ time, but I am not sure which. Do you happen to know? If it was Moses’ time, did the religious calendar come in before or after Exodus 19?
The religious calendar came into existence just before the first Passover, which was followed by the Exodus from Egypt. According to Exodus 12:1-2 the Lord changed the Hebrew calendar to begin in the spring, whereas it had previously begun in the fall. The Jews maintained the original calendar which was based on growing seasons and superimposed the religious calendar over it. This explains why the Jewish New Year comes in the seventh month (Leviticus 23:23-25).