“To be human is to be aware of the passage of time; no concept lies closer to the core of our consciousness.”
– Dan Falk,  In Search of Time: The Science of a Curious Dimension

It is easy for me to imagine the earliest humans watching the motions of the sun and stars and wondering.  they knew the light came and went and that they slept and awakened and they must have had some sense of Time.

Early civilizations all had their way of marking the passage of days. At some point they divided those days into parts – sunlight and darkness; before high noon, after noon and after sunset; eventually, even smaller divisions.

And then, after figuring out the pattern to the seasons, came a calendar for mapping out the year.

and some good reads about time, is

In  Dan Falk’s book, he says that the tracking of celestial motions may have occurred as early as the Paleolithic period. But it took civilization and agriculture-based urban settlements with true writing systems for people to show us that they were accurately recording days, months, and years.

Some marked the time by lunar cycles which equal 29.5306 days.  Some marked the time by following the solar year of  365.2422 days. Unfortunately, even with less exacting measurements, the two cycles don’t fit together evenly.  Falk mentions that in the fifth century B.C., Aristophanes had the Moon complaining in his play, The Clouds, that the days refused to keep pace with her phases!

The ancient Sumerians just rounded the length of the month up to 30 days for a 360-day year. Pretty close. If you go lunar, and assume exactly 12 months in a year, you get a 354 day year. Only 11 days off.

The problem is that if you use that lunar calendar, the new year is 11 days earlier than the year before. Give that calendar 16 years of use and your midsummer celebration becomes a midwinter celebration.

The problem wasn’t unknown to the ancients. The Babylonians figured it out and used the sighting of the crescent moon in the western sky to start a month (this is still followed in Muslim nations to this day) and so that the calendar stayed in solar sync, the Babylonians employed a cycle in which seven 13-month years alternated with 12 years of just 12 months.

They called this 19-year unit a Metonic cycle, after the Greek astronomer Meton of Athens, who discovered that 235 lunar months is almost exactly the same interval as 19 solar years. If you used his calendar, you’d only be off by one day every 219 years. The Jewish calendar is closely modeled on the Babylonian one and is also built on the 19-year Metonic cycle.