calculate from ISO week calendar date (year, week, weekday) into gregorian calendar date (year, month, date)

dateFromIsoWeek.d

/* hoping to get this into phobos: https://github.com/dlang/phobos/pull/7420 */

Date dateFromISOWeek(short isoWeekYear, ubyte isoWeek, DayOfWeek weekday) @safe pure
{
	immutable adjustedWeekday = weekday == DayOfWeek.sun ? 7 : weekday;
	immutable dayOffset = (isoWeek - 1) * 7 + adjustedWeekday;

	Date date = Date(isoWeekYear, Month.jan, 3);
	immutable startOfYear = date.dayOfWeek;
	return date + days(dayOffset - startOfYear);
}