JS Jquery 根据两个日期计算相差月数天数 兼容二月份28天29天

var start_date = '2019-08-21';
var end_date = '2020-09-29';
用法：
var month_day = getMonthAndDay(new Date(start_date), new Date(end_date));
返回：{month:13,day:9}function getMonthAndDay(startdate, enddate) {if (startdate.valueOf() > enddate.valueOf()) {var tmpdt = enddate;enddate = startdate;startdate = tmpdt;}var month = 0;var day = 0;enddate = getNextDay(enddate);if (enddate.getTime() >= startdate.getTime()) {month = (enddate.getFullYear() - startdate.getFullYear()) * 12 + enddate.getMonth() - startdate.getMonth();day = enddate.getDate() - startdate.getDate();if(day < 0){var lastmonthdaynum = getlastmonthDay(enddate);day += lastmonthdaynum;if(lastmonthdaynum < 30 && startdate.getDate() > 28){day += startdate.getDate() - lastmonthdaynum - 1;}month--;}} else {if (startdate.getTime() == getlastDay(startdate)) {if (enddate.getTime() == getlastDay(enddate)) {month = enddate.getFullYear() - startdate.getFullYear() * 12 + enddate.getMonth() - startdate.getMonth();day = 0;} else {month = enddate.getFullYear() - startdate.getFullYear() * 12 + enddate.getMonth() - startdate.getMonth() - 1;day = startdate.getDate();}} else {if (enddate.getTime() == getlastDay(enddate)) {month = enddate.getFullYear() - startdate.getFullYear() * 12 + enddate.getMonth() - startdate.getMonth();day = 0;} else {month = enddate.getFullYear() - startdate.getFullYear() * 12 + enddate.getMonth() - startdate.getMonth() - 1;var lastmonthday = getlastmonthDay(enddate);day = lastmonthday - startdate.getDate() + enddate.getDate();}}}return {"month" : month,"day" : day};
}
//得到上一个月有多少天
function getlastmonthDay(date) {date.setDate(1);date.setDate(date.getDate() - 1);return date.getDate();
}
/获取指定日期的后一天日期
function getNextDay(d){d = new Date(d);d = +d + 1000*60*60*24;d = new Date(d);var res = d.getFullYear()+"-"+(d.getMonth()+1)+"-"+d.getDate();return new Date(res);
}
//获取 date 所在的月有多少天
function getlastDay(date) {date.setMonth(date.getMonth() + 1);date.setDate(1);date.setDate(date.getDate() - 1);return date.getDate();
}