The synodic month is a period of 29.5 days in which the phases of the Moon returns.
It was used to measure time at least since the time of the Sumerians (7,000 years ago): every time when the crescent Moon appears as a slender arc in the western evening sky, a new month begins.
This notation is still in effect regarding the phases of the Moon - below you have a picture of a 20 days and 19 hours old Moon.

Canon 700D + 14” telescope

4 pictures mozaic in PS.