Try 'cal 9 1752' on any machine running UNIX operating system. The output
 looks something like...

         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

 Where have days 3-13 (inclusive) gone? The reason for this is a historical
 one, which goes as follows...

 THE AMAZING 80% YEAR OF 1752

 The modern calendar is a compilation of usages dating from the days of Julius
 Caesar, when the Greek astronomer and mathematician Sosigenes devised for Rome
 a calendar only essentially familiar to us as our present-day calendar. It was
 during the late 16th century that work of the 6th century Anglo-Saxon monk,
 Bede, was submitted to Pope Gregory XIII who accepted the calculations and
 made the decision to issue a more accurate calendar which ultimately was
 accepted.

 The Gregorian calendar was adopted by most Roman Catholic countries. Many
 protestant countries did not accept the new calendar until the 18th century,
 including Britain, which meant America as well. The British finally accepted
 the Gregorian calendar for itself and all its possessions effective March 25,
 1752. But, a remarkable 2-step change was made which is little realized today.

 It is most important that the reader understand that the British used to
 change the year number on March 25th and not January 1st as we do today.  On
 March 24, 1751 the next day would be March 25, 1752, advancing the year as
 usual.  But, a somewhat confusing phenomenon occurred, and for present day
 imaginations, comprehension requires close attention.

 1. First, it was decreed that 1752 should end with December 31st and not be
    carried on to the next March 25th.

 2. Second, it was also decreed that the arrival of September 2, 1752 should be
    called September 14, 1752. This should be Wednesday. Sept 2, 1752 was
    followed by Thursday Sept 14. Sept 2 existed. Sept 3-13 (11 days) did not.
    Sept 14 existed. Remember the slogan of the protestors "Give us back our 11
    days". For the sake of clarity we explain. The period of January 1 - March
    24, 1751 was the end of an epoch. The year of 1752 began on March 25th and
    ended with December 31, 1752. Thus the earlier days of 1752 never existed,
    as the deleted days of September 2-13 also never existed. The year 1752 was
    a very short year, 72 days shorter, in fact.

 Of course, dates may be recomputed from the old to the new calendars. For
 instance, George Washington's birthday was February 11, 1731 as far as his
 mother was concerned. Today, we must reckon his birthday as February 22, 1732
 in consideration of the new calendar. To put it in perspective, if we count
 backwards from today the actual number of days since G. W. was born, we would
 come up with February 22, 1732 using the present-day calendar.

 To convert from the old Julian to the new Gregorian calendar, one must add 10
 to 13 days to the old date, and sometimes change the year one extra when the
 date considered falls within the period January 1 - March 24. This is
 crucially important to those interested in genealogy and historical research.
 Documented dates before March 25, 1752 do not necessarily always corrobate a
 stated period of time. And, references to any New Year's Day before 1752, in
 Great Britain, meant March 25th.