DateTime.daysInMonth

assert(DateTime(Date(1999, 1, 6), TimeOfDay(0, 0, 0)).daysInMonth == 31);
assert(DateTime(Date(1999, 2, 7), TimeOfDay(19, 30, 0)).daysInMonth == 28);
assert(DateTime(Date(2000, 2, 7), TimeOfDay(5, 12, 27)).daysInMonth == 29);
assert(DateTime(Date(2000, 6, 4), TimeOfDay(12, 22, 9)).daysInMonth == 30);