yyyy-mm-dd 向前取一个月时间 (传入参数是 yyyy-mm-dd 字符串 返回值同)

function getPreMonth(date) {
    var arr = date.split('-');
    var year = arr[0];
    var month = arr[1];
    var day = arr[2];
    var days = new Date(year, month, 0);
    days = days.getDate(); 
    var year2 = year;
    var month2 = parseInt(month) - 1;
    if (month2 == 0) {
        year2 = parseInt(year2) - 1;
        month2 = 12;
    }
    var day2 = day;
    var days2 = new Date(year2, month2, 0);
    days2 = days2.getDate();
    if (day2 > days2) {
        day2 = days2;
    }
    if (month2 < 10) {
        month2 = '0' + month2;
    }
    var t2 = year2 + '-' + month2 + '-' + day2;
    return t2;
}

　　yyyy-mm-dd 转时间戳 

var time = Date.parse(new Date("2019-11-27")) / 1000;

　　判断时间跨度 st en (伪代码)

(en - st) compare with (86400 * days)