(mysql.info) mysql-calendar

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 12.6 What Calendar Is Used By MySQL?
 ====================================
 
 MySQL uses what is known as a proleptic Gregorian calendar.
 
 Every country that has switched from the Julian to the Gregorian
 calendar has had to discard at least ten days during the switch.  To
 see how this works, consider the month of October 1582, when the first
 Julian-to-Gregorian switch occurred:
 
 Monday     Tuesday    Wednesday  Thursday   Friday     Saturday   Sunday
 1          2          3          4          15         16         17
 18         19         20         21         22         23         24
 25         26         27         28         29         30         31
 
 There are no dates between October 4 and October 15. This discontinuity
 is called the cutover. Any dates before the cutover are Julian, and any
 dates following the cutover are Gregorian. Dates during a cutover are
 non-existent.
 
 A calendar applied to dates when it wasn't actually in use is called
 proleptic. Thus, if we assume there was never a cutover and Gregorian
 rules always rule, we have a proleptic Gregorian calendar. This is what
 is used by MySQL, as is required by standard SQL. For this reason,
 dates prior to the cutover stored as MySQL `DATE' or `DATETIME' values
 must be adjusted to compensate for the difference. It is important to
 realize that the cutover did not occur at the same time in all
 countries, and that the later it happened, the more days were lost. For
 example, in Great Britain, it took place in 1752, when Wednesday
 September 2 was followed by Thursday September 14. Russia remained on
 the Julian calendar until 1918, losing 13 days in the process, and what
 is popularly referred to as its `October Revolution' occurred in
 November according to the Gregorian calendar.

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