SysTime.isoWeek

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

auto st = SysTime(Date(1999, 7, 6));
const cst = SysTime(Date(2010, 5, 1));
immutable ist = SysTime(Date(2015, 10, 10));

assert(st.isoWeek == 27);
assert(cst.isoWeek == 17);
assert(ist.isoWeek == 41);