Decrementing 'day' on Jan 1 results in NaN

[kastentx]

tc39-proposal-temporal/lib/civil/shared.mjs

Lines 38 to 41 in db3346f

	while (day < 1) {
	month -= 1;
	day = daysInMonth(year, month) - day;
	if (month < 1) { month = 12; year -= 1; }

Issue arises when day and month are both 1, and the day is decremented by 1, e.g:

MyCivilDate.plus({ days: -1 })

This causes daysInMonth to be called with a month value of 0 which is then decremented by 1 to get the index, which fails.

[sosoba]

Is it correct to decrement year:

year -= 1;

without checking? Remember that gregorian callendar doees not has 0 year and 1 - 1 should be -1.