It's God's measure,
since He created the moon.
The sun is a timekeeper as well,
as are the stars:
14 Then God said, “Let there be lights in the firmament of the heavens to divide the day from the night;
and let them be for signs and seasons,
and for days and years….Gen. 1
Stars measure seasons,
the sun measures years,
the moon measures months.
However,
the earth's rotation measures the day.