[Bug 237259] cal(1) command outputs wrong data in September 1752

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=237259

--- Comment #4 from Conrad Meyer <cem at freebsd.org> ---
util-linux's cal(1) is unfortunately not an independent implementation for
comparison purposes — it's at least derived from BSD cal (and produces same
result, without extra options).

Some more reading from our cal.1 page:

     -p      Print the country codes and switching days from Julian to
             Gregorian Calendar as they are assumed by ncal.  The country code
             as determined from the local environment is marked with an
             asterisk.

     -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.
...
BUGS
     The assignment of Julian–Gregorian switching dates to country codes is
     historically naive for many countries.

And from util-linux' cal.1:

       Two different calendar systems are used, Gregorian and Julian.  These
are nearly
       identical systems with Gregorian making a small adjustment to the 
frequency  of
       leap years; this facilitates improved synchronization with solar events
like the
       equinoxes.  The Gregorian calendar reform was introduced in 1582, but
its  adop‐
       tion  continued  up  to  1923.   By default cal uses the adoption date
of 3 Sept
       1752.  From that date forward the  Gregorian  calendar  is  displayed; 
previous
       dates use the Julian calendar system.  11 days were removed at the time
of adop‐
       tion to bring the calendar in sync with solar events.  So Sept 1752 has
a mix of
       Julian  and  Gregorian  dates  by which the 2nd is followed by the 14th
(the 3rd
       through the 13th are absent).

       Optionally, either the proleptic Gregorian calendar or the Julian 
calendar  may
       be used exclusively.  See --reform below.

(N.B., our cal does not have this --reform date option.)

       --reform val
              This  option  sets  the  adoption  date of the Gregorian calendar
reform.
              Calendar dates previous to reform use the Julian calendar system.
 Calen‐
              dar  dates  after reform use the Gregorian calendar system.  The
argument
              val can be:

              · 1752 - sets 3 September 1752 as the reform  date  (default).  
This  is
                when the Gregorian calendar reform was adopted by the British
Empire.

              · gregorian  -  display  Gregorian  calendars  exclusively.  This
special
                placeholder sets the reform date below the smallest year that 
cal  can
                use;  meaning  all  calendar output uses the Gregorian calendar
system.
                This is called the proleptic Gregorian calendar, because dates
prior to
                the calendar system's creation use extrapolated values.

              · iso - alias of gregorian.  The ISO 8601 standard for the
representation
                of dates and times in information interchange requires using 
the  pro‐
                leptic Gregorian calendar.

              · julian  -  display  Julian  calendars exclusively.  This
special place‐
                holder sets the reform date above the largest year that  cal 
can  use;
                meaning all calendar output uses the Julian calendar system.

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