Priests in ancient Babylon divided the sky in a belt of 12 sectors, the Zodiac. Each sector, each group of stars, indicated the orbit of the sun, which seemed to move from constellation to constellation. They discovered that every 30 days the moon was full, so they divided the year in 12 x 30 = 360 days. Pretty soon though, they discovered a shortage in days. The error of 5 days a year mounted very rapidly: a full month in 6 years. They corrected for this by creating a 13 month year every 6th year. The Egyptians also used a 360-day year. However, they corrected this by adding 5 days of festivities at the end of each year.
Because a solar year is about 365 1/4 days long, and the current division didn’t match the seasons, Ptolomeus (appr. 240 BC) suggested to add one day every 4 years. Ptolomeus got in trouble with the priests, but 200 years later, Julius Caesar made it official. They were getting close. The actual solar year is 365 days, 5 hours, 48 minutes and 14 seconds, slightly shorter than the Julian calendar with an error of 3 days per 400 years.
In 1582, Pope Gregorius XIII, together with some scholars, decided that every turn of the century that was dividable by 400 was a leap-year, those that are not (1800, 1900, 2100), are normal years. This leaves the difference with the actual solar year 26 seconds, i.e. 1 day per 3323 years. We still use the Gregorian calendar.
During the Revolution, the French tried to reinforce the 12 months/ 360 days – year, using a decimal timescale, adding 5 days of festivities at the end of the year. A day consisted of 10 hours of 100 minutes. Minutes were divided in 100 seconds. 10 days made a “week”, called a “dekade”. There were 30 days in a month. The republican calendar was not a success and lasted only from 1793 till 1805.
The Magic Number in the early calendar was 60. The zodiac was divided in 360 days/degrees; days are divided in 2 x 12 units, 60 minutes per “unit, 60 seconds per minute.
(After A. Holleman in “De Rikketik”, magazine of the Dutch society for watch and clock collectors, issue 1 of 1995)