Old physical calendars fit several general purposes: mainly planting cycles, religious observance based on the mythology that grew from nature's cycles, and reference dates for history of each ruler's important life events and their dynasties' reigns.
But don't forget, even before these inscribed calendars came into existence, celestial events were observed with notches in rocks or notches where mountains form peaks or Vs. Everything in nature has an observable cycle. Seasons, seasonal rains and flooding, dry periods, ripening of foods, animal birthing and migrations, sun and moon cycles, positions of constellations; all these make cycles for all beings to observe.
I would say that nature developed the calendar, we just notice it, quantify it, and live our lives by it in useful (and meaningless) ways.
