Our current calendar is based on the movements of the Earth around the Sun. The jewish calendar is based on the movements of Saturn in relation to the Earth and the Sun. The muslim calendar is based on the movements of the moon around the Earth. Belief systems have always been based around simple natural events - like the changing of seasons and what not - and deities always ascribed to natural phenomenon.  However, these are different beliefs in that instead of planting crops for physical nourishment, they murder and torture for spiritual nourishment.