The last day in the month that this Date is in.
assert(Date(1999, 1, 6).daysInMonth == 31); assert(Date(1999, 2, 7).daysInMonth == 28); assert(Date(2000, 2, 7).daysInMonth == 29); assert(Date(2000, 6, 4).daysInMonth == 30);
See Implementation
The last day in the month that this Date is in.