(c. 15,000 B.C.E.)
The passage of time was noted by early people.
The earliest lunar calendar that has been discovered was created circa 15,000 B.C.E. in the Lascaux caves in southwest France. A vast empty square that may symbolise a clear sky is followed by a number of series of spots that collectively reflect half of the moon's nearly monthly cycle.
A lunar calendar is based on the phases of the moon and counts months (which have a duration of 29.530588 days). Twenty-nine and thirty days make up a month. To maintain pace with the real moon phase, additional days are occasionally added in addition to the alternate schedule.
In many parts of the ancient world, the lunar calendar was used for religious observance. The lunar calendar is difficult for agriculture since it makes no allowance for seasonal fluctuations in temperature, length of day, plant growth, animal migration, and mating that occur every year. There are 10.88 days left in the lunar month after twelve divisions into the solar year.
Around 440 B.C.E., Meton of Athens discovered that 234.997 lunar months were equal to 19 solar years. This resulted in a nineteen-year Metonic cycle with thirteen lunar months in years three, five, eight, eleven, thirteen, sixteen, and nineteen, and twelve months in all the other years.