subISOWeekYears and addISOWeekYears change date's time, even when adding/subtracting 0 ISO weeks

[gregsadetsky]

repro:

const originalDT = parseISO("2023-03-10T00:00:00Z");
// adding 0 iso week years -- prints -68400000
console.log(addISOWeekYears(originalDT, 0) - originalDT);
// substracting 0 iso week years -- prints -68400000
console.log(subISOWeekYears(originalDT, 0) - originalDT);

as far as I can see, the source of this problem is that:

• subISOWeekYears uses addISOWeekYears internally, so they both exhibit the same issue
• addISOWeekYears usessetISOWeekYear internally
• setISOWeekYear uses startOfISOWeekYear internally
• startOfISOWeekYear uses startOfISOWeek internally
• startOfISOWeek uses startOfWeek internally
• finally, startOfWeek calls .setHours(0, 0, 0, 0) internally to set the date of the object that's returned through the chain described above

Thanks!