已知当前月，获取当前月的最后一天

getLastDay(year, month) {
	let newYear = year,
		newMonth = month++;
	if (month > 12) {
		newMonth -= 12;
		newYear++;
	}
	let newDate = new Date(newYear, newMonth, 1);
	return new Date(newDate.getTime() - 1000 * 60 * 60 * 24).getDate();
}