JS: Непонятная ошибка.

function AUTOCOMPLETE() {
	let date = new Date(document.getElementById('DATE1').value);
	let month = date.getMonth()+1;
	let year = date.getFullYear();
	let sdate = document.getElementById('DATE2');
	let tdate = document.getElementById('DATE3');
	if (month<10) {
		sdate.value = 'year-0month-01'.replace('year', year-1).replace('month', month);
	} else {sdate.value = 'year-month-01'.replace('year', year-1).replace('month', month);}
	if (month<11) {
		tdate.value = 'year-0month-31'.replace('year', year).replace('month', month-1);
	} else {tdate.value = 'year-month-31'.replace('year', year).replace('month', month-1);}
	if (month==1) {
		month = 12;
		tdate.value = 'year-month-31'.replace('year', year-1).replace('month', month);
	}
}

The specified value "2021-06-31" does not conform to the required format, "yyyy-MM-dd".