Notice 11 days missing? This isn't a bug. It is a bug if you don't get something similar to the above output.
In case you can't get the answer from your man(ual) page (try "man cal"), here's why:
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 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."
From http://en.wikipedia.org/wiki/Cal_(Unix)