https://susam.in/maze/cal-9-1752.html

cal 9 1752

By Susam Pal on 14 Sep 2004

A very interesting thing I came across recently while learning Unix
is an apparent gap in the calendar for Sep 1752. Here is how the cal
output for this month appears on a Unix or Linux system:

$ 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

In the SunOS 5.9 manual page for cal(1), this gap is explained as
follows:

    An unusual calendar is printed for September 1752. That is the
    month 11 days were skipped to make up for lack of leap year
    adjustments. To see this calendar, type: cal 9 1752

Similarly, FreeBSD 5.0 has the following note in its manual page for
cal(1):

    -s country_code

    Assume the switch from Julian to Gregorian Calendar at the date
    associated with the country_code. If not specified, ncal tries to
    guess the switch date from the local environment or falls back to
    September 2, 1752. This was when Great Britain and her colonies
    switched to the Gregorian Calendar.

On investigating this switch from Julian to Gregorian calendar, I
learnt that the Julian calendar was introduced by Julius Caesar in 46
BC and it remained in effect across most of the Western world from 45
BC to 1582. It assumed an average year to be 365.25 days long.
However, the actual solar year has been known to be around 365.2422
days since the 17th century. Although the difference appears to be
too small, it leads to an error of adding 1 extra day every 128
years. To reduce this error, the Gregorian calendar was introduced in
October 1582 by Pope Gregory XIII. It assumed an average year to be
365.2425 days long.

The new calendar was adopted in some European countries where 4 Oct
1582 was followed by 15 Oct 1582 thereby skipping 10 days in between.
However, it took as long as September 1752 for the new calendar to be
adopted by Britain. In Great Britain and the British Empire, 2 Sep
1752 was followed by 14 Sep 1752 and that is the gap of 11 days we
see in the cal 9 1752 output.

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