import std.datetime.date : DateTime;

assert(SysTime(DateTime(1999, 7, 6, 9, 7, 5)).day == 6);
assert(SysTime(DateTime(2010, 10, 4, 0, 0, 30)).day == 4);
assert(SysTime(DateTime(-7, 4, 5, 7, 45, 2)).day == 5);