The civil day in ancient cultures was made up of "Watches". The length of the watch varied with the season, and were called seasonal or temporal hours. They were related to the length of the Sun’s time above the horizon. This method was known as far back as 1800 BC and was used until the end of the 13th century AD in Europe. At that time it became inconvenient to use because of the invention of the mechanical clock. The seasonal method was uneven and the mechanical clock had an even 12 hours for day and 12 hours for night. The 12 comes from Babylonian 2 x 12 = 24 (5 x 12 = 60). They did this rather than using the base 10. This is known as the Sumerian sexagesimal system.

And it has carried to this day, we use 60 SECONDS in a MINUTE, 60 minutes in an HOUR. Today the system used for the calendar is the mean solar DAY, which is 24 hours 3 minutes 56.55 seconds.