Want to see a cool trick and learn some history? Go into terminal (if you are using UNIX or Mac OS X) and type in “cal 9 1752″ at the prompt. What will be displayed is a calendar of September 1752. The odd thing is that there is an entire 11 days missing from the month (Sept. 3rd-13th), as seen here:
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.