js计算年月加减月份

• time 的格式为"yyyy-MM"
• num 的格式为n，n为正数表示加月份，为负数表示减月份

export function calcMonths(time, num) {
	let arr = time.split("-"); // 将 "yyyy-MM" 日期分解为 ["2021","09"]
	let year = parseInt(arr[0], 10);
	let month = parseInt(arr[1], 10);
	month = month + num;
	if (month > 12) {
		let newYear = parseInt((month - 1) / 12, 10);
		month = month % 12 == 0 ? 12 : month % 12;
		year += newYear;
	} else if (month <= 0) {
		month = Math.abs(month);
		let newYear = parseInt((month + 12) / 12, 10);
		let n = month % 12;
		if (n == 0) {
			year -= newYear;
			month = 12;
		} else {
			year -= newYear;
			month = Math.abs(12 - n);
		}
	}
	month = month < 10 ? "0" + month : month;
	return year + "-" + month;
}