September 1752

This particular month has something special. If you type cal 9 1752 on any Unix command line to get a calendar of this particular month you’ll get this output:

   September 1752
 S  M Tu  W Th  F  S
       1  2 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30

Not a typical calendar by any means! Where have gone the days between September 2 and 14? Answer: September 1752 is the month Britain switched from the Julian calendar to the Gregorian calendar.

This means that dates prior September 1752 should be calculated according to the Junian calendar right? This is where it gets confusing: not all countries have switched at the same time.

So when you have an old date, you should make sure you know which calendar to use before doing any calculation with it.

(via email from Henri Sivonen on WhatWG mailing list)