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function formSetDay(){

	const $changeMonth=$('.js-changeMonth');
	const $select = $('.js-changeDay');
	const selectedDate = $select.val();

	const today = new Date();
	const year=today.getFullYear();

	const lastday = new Date(year, $changeMonth.val(),0).getDate();

	$select.empty();

	for (let i = 1; i <= lastday; i++) {
		const $option =$('<option/>');
		$option.val(i).text(i);
		$option.prop('selected',i === selectedDate);
		$select.append($option);
	}

}

$('.js-changeMonth').change(function(){
	formSetDay();
});