SysTime.daysInMonth

import core.time;
import std.datetime.date : DateTime;

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