If time were not there, there wouldn’t be possibilities to describe changes: movement, evolution and transitoriness. Time is a relative size. What does a second mean? Until a few decades ago, the second was considered the 86,400th part of a particular mean solar day. Meanwhile, this definition is too inaccurate for us. Today, the number of oscillations (9192631770 Hertz) of a cesium-133 atom unaffected by external fields is valid for one second.

The calendar is the time grid of every life. The different calendars represent a chronological division of the year according to astronomical or tropical criteria. The astronomical division of the year is made according to moon phases and the tropical according to the annual recurring seasons. However, the lunar year (354.367082 days) is almost 11 days shorter than the solar year (365.242199 days) so that the Mohammedan and Gregorian calendars drift apart.

In the Julian calendar introduced by Caesar three consecutive years were counted with 365 days each and the fourth year, the leap year, with 366 days. This resulted in a mean annual length of 365.25 days, which was a bit too long compared to the tropical annual division and brought up in the 16th century a shift of the beginning of the year by 10 days. With the calendar reform of Pope Gregory the XIII in 1582 and the introduction of the Gregorian calendar, which is still valid today, the shift of these 10 days has been reversed. Each calendar year divisible without remainder by 4 is a leap year with 366 days, whereby every 400 years 3 leap years have to be precipitated, namely those years of the centuries, which cannot divide integer by 400 like 2100, 2200 and 2300. The years 2000 and 2400 have a February 29th.

From one full moon to another, 29 days, 12 hours, 44 minutes and 2.8 seconds pass. Most moon-phase clocks or watches have 59 teeth on a gear and two opposite moons on the lunar disc, only one moon is visible whereas the other is concealed behind the mask (59/2 = 29.5 days). The deviation of minus 44 minutes and 2.8 seconds adds up to a day in 32 months..