Ancient Calendars and Time Keeping Mechanisms

It's interesting to read about Stonehenge and wonder why so many people over the span of several thousand years were attracted to this one place. It seems to be a very religious spot.

Other calendar systems I've been thinking about are lunar and solar calendars. By watching the changing sun and the changing moon, people could know when to plant and harvest - without some sort of system, people wouldn't have had food. It makes me wonder what other calendars existed that we don't know about ...