cal 9 1752
You can also use this to troll developers that are doing anything that has a calendar.
>The Gregorian reformation was adopted by the Kingdom of Great Britain, including its possessions in North America (later to become eastern USA), in September 1752. As a result the September 1752 cal shows the adjusted days missing. This month was the official (British) adoption of the Gregorian calendar from the previously used Julian calendar. This has been documented in the man pages for Sun Solaris as follows. "An unusual calendar is printed for September 1752. That is the month when 11 days were skipped to make up for lack of leap year adjustments." The Plan 9 from Bell Labs manual states: "Try cal sep 1752." [0]
[0] http://en.wikipedia.org/w/index.php?title=Cal_(Unix)#Feature...
For us Julian calendar folks, there's upsides and downsides. We get to celebrate Christmas late, but my kid will celebrate both (since my wife is of a different religion). And of course, Easter falls on Cinco de Mayo this year (but we had remember to buy enough egg coloring well in advance)! :)
cal 2 1712
should show February 30th...