SysTime.dayOfYear

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

assert(SysTime(DateTime(1999, 1, 1, 12, 22, 7)).dayOfYear == 1);
assert(SysTime(DateTime(1999, 12, 31, 7, 2, 59)).dayOfYear == 365);
assert(SysTime(DateTime(2000, 12, 31, 21, 20, 0)).dayOfYear == 366);