Forwarding Address: OS X discusses the incredibly useful cal command line tool.
Mankind has had a notoriously weird notion of time over the ages. It wasn't until fairly recently that clocks and calendars were synchronized around the world. While time zones are much more complex, the day/month/year calendar has its share of oddities.
cal's implementation, thus, required knowledge of such events to be built-in. From the man page:
The Gregorian Reformation is assumed to have occurred in 1752 on the 3rd
of September. By this time, most countries had recognized the reforma-
tion (although a few did not recognize it until the early 1900's.) Ten
days following that date were eliminated by the reformation, so the cal-
endar for that month is a bit unusual.
% cal 9 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