The original calendar was given to humanity by God back in the time of Adam and Eve, which was first discussed in Genesis 7:11 (KJV), “In the six hundredth year of Noah’s life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened.” It is assumed since Moses wrote the first 5 books of the Old Testament and he wrote them in Hebrew, the calendar system Moses was writing about was indeed the Jewish calendar.