The Amazing Disappearing Days

The UNIX cal command takes into account events like leap year. For instance, if you want a calendar for August 2010 or February 2012, the UNIX cal command will produce the correct results.

Let’s try an experiment. Execute the following UNIX/LINUX command.

cal 9 1752

The output looks like this

September 1752

Su Mo Tu We Th Fr Sa
       1  2 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30

Days 3-13 are gone! What happened to those 11 days?


The modern calendar is a compilation of many usages dating from the days of Julius Caesar when the Greek astronomer and mathematician Sosigenes devised a calendar for Rome that is essentially the one that is familiar to us as our present-day calendar. During the late 16th century, Pope Gregory XIII accepted 6th-century Anglo-Saxon monk Bede's calculations and issued a more accurate calendar. Most Roman Catholic countries adopted the Gregorian calendar. Many Protestant countries did not accept the new calendar until the 18th century, including Great Britain, which also meant America.

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.

Great Britain and all its possessions finally accepted the Gregorian calendar, effective March 25, 1752. The government required a remarkable 2-step change.

First, the government decreed that 1752 ended on December 31st and not be carried on to the following March 25th.

For clarity, January 1 through March 24, 1751, ended an epoch. 1752 began on March 25th and ended on December 31, 1752. The earlier days of 1752 never existed, as the deleted days of September 3-13 also never existed. The year 1752 was very short -- 72 days.

Of course, we can recompute dates from the old to the new calendars. For instance, George Washington's birthday was on February 11, 1731, as far as his mother was concerned. Considering the new calendar today, we must reckon his birthday as February 22, 1732. Suppose we count backward from today the actual number of days since Washington was born. In that case, we will 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 when the date considered falls between January 1 and March 24. This is crucially important to those interested in genealogy and historical research. Documented dates before March 25, 1752, do not always correlate with a stated period. References to any New Year's Day before 1752 in Great Britain meant March 25th.

Second, the government also decreed that September 3, 1752, was September 14, 1752. Thursday, September 14, followed Wednesday, September 2, 1752. September 2nd and 14th existed, but September 3rd through the 13th (11 days) did not.

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