You can also do
Calendar.Month()->minute(-1);
(1) Result: Minute(Sat 30 Apr 2011 23:59 CEST)
to actually get the last minute of the month.
Or, for the first one:
Calendar.Month()->minute(0);
(2) Result: Minute(Fri 1 Apr 2011 0:00 CEST)
This works for weeks too:
Calendar.Month()->week(-1);
(3) Result: Week(w17 2011)