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Although the number of days in each month varies from 31 to 30, or even 28, and we sometimes have to resort to the "Thirty days hath September ..." rhyme to remember the days in a particular month, it's a fairly regular system, once you get used to it.
The ancients knew the length of the year fairly accurately, as well as the length of the lunar cycle.
For further information on calendars, see Richards, E.
We refer to a yyddd date format (yy = year, ddd=day) as a 'Julian Date' - this is the common term for such a date in mainframe and other circles.
Note that the time scale that is the basis for Julian dates is Universal Time, and that 0h UT corresponds to a Julian date fraction of 0.5.
It is assumed that 7-day weeks have formed an uninterrupted sequence since ancient times.
Most astronomy texts explain that it is based on the number of days that have elapsed since noon universal time (UT), 1 January 4713 BCE (before current era, or B. If you think that "Julian" refers to the Julian calendar, named after the Roman emperor Julius Caesar, you are both right and wrong. This system was named after Julius Caesar, but not the one who ruled Rome and was assassinated by Brutus and others.
When rulers changed, sometimes the calendar did, too.We add an extra day every four years to the usual 365, to account for the approximate 365.25 days it takes for Earth to orbit the sun.We divide the year into 12 months, based on the approximate 29.5 days of each lunar cycle.The Smithsonian Astrophysical Observatory introduced the 'Modified Julian Date' to record the orbit of the Sputnik satellite in 18 bits.A yyddd (or similar format such as yyyyddd, yy-ddd) date is more correctly called an ordinal date.