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If you type cal 9 1752 in a Linux terminal you will get strange output. For example:

[max@avi ~]$ cal 9 1752

   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

Did you notice the date from 3 to 13 is missing? Why is this so? I am using CentOS 6.2.

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Fun fact: man cal explains that...stumbled upon that while integrating it into my conky script. Or at least some man-pages do... – Bobby Jul 23 '12 at 10:53
up vote 14 down vote accepted

It is not a problem, these dates were skipped in September 1752.

Britain and the British Empire (including the eastern part of what is now the United States) adopted the Gregorian calendar in 1752, by which time it was necessary to correct by 11 days. Wednesday, 2 September 1752 was followed by Thursday, 14 September 1752.


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The guy that originally wrote the "cal" command on some old Version 7 machine had an off-by-one error in his code. This showed up as some erroneous output when a malloc'd variable overwrote 12 extra bytes with zeroes, thus leading to the strange calendar output seen above.

Now, nobody in his right mind really cares about the calendar for September 1752. Even the idea of the year 1752 does not exist under UNIX, because time did not begin for UNIX until early 1970. As a result, nobody even knew that "cal" had this error until much later. By then there were thousands of copies of "cal" floating around, many of them binary-only. It was too late to fix them all.

So in mid-1975, some high-level AT&T officials met with the Pope, and came to an agreement. The calendar was retroactively changed to bring September 1752 in line with UNIX reality. Since the calendar was changed by counting backwards from September 14, 1752, none of the dates after that were affected. The dates before that were all moved by 12 days. They also fixed the man page for "cal" to document the bug as a feature.

The 11 days from September 3 to September 13 were simply gone from the records. They searched the history books and found that fortunately nothing of much significance happened during those 11 days.

Overall, this whole incident was pretty much a non-event. One science fiction author later heard about it, and blew the thing up into a full-length work of science-fiction called "The Lathe of Heaven", a book that in my opinion bears little resemblence to what really happened.

original source

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The way this answer is written hides the fact that it's a straight cut and paste from a humor site. – Ward Jul 23 '12 at 14:59

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