Measuring Time

Basic concepts of TIME are from astronomy

The calendar:

The Gregorian calendar is based on a cycle of 400 years, or 146,097 days. Since 146,097 is evenly divisible by 7, the Gregorian calendar exactly repeats after 400 years. Dividing 146,097 by 400 yields an average length of 365.2425 days per calendar year, which is a close approximation to the length of the tropical year.  The Gregorian calendar accumulates an error of one day in about 2500 years.   (see, especially, &2.3 in Calendars and  more on calendars, the Date of Easter )

Interesting questions: