js日期处理多少/月/天以前

 var date = new Date();
    var year = date.getFullYear();
    var month = (date.getMonth() + 1) > 10 ? date.getMonth() + 1 : '0' + (date.getMonth() + 1);
    var day = date.getDate() > 10 ? date.getDate() : '0' + date.getDate();
    var nowdatestr = year + "-" + month + "-" + day;
    var darr1="";

    //近3天
    function getDay(day) {
        var today = new Date();
        var targetday_milliseconds = today.getTime() + 1000 * 60 * 60 * 24 * day;
        today.setTime(targetday_milliseconds); //注意，这行是关键代码
        var tYear = today.getFullYear();
        var tMonth = today.getMonth();
        var tDate = today.getDate();
        tMonth = doHandleMonth(tMonth + 1);
        tDate = doHandleMonth(tDate);
        return tYear + "-" + tMonth + "-" + tDate;
    }

    function doHandleMonth(month) {
        var m = month;
        if (month.toString().length == 1) {
            m = "0" + month;
        }
        return m;
    }

    //近7天
    $("#sevenday").click(function () {
        var oneweekdate = new Date(date - 6 * 24 * 3600 * 1000);
        var year1 = oneweekdate.getFullYear();
        var month1 = oneweekdate.getMonth() + 1 > 10 ? oneweekdate.getMonth() + 1 : '0' + oneweekdate.getMonth() + 1;
        var day1 = oneweekdate.getDate() > 10 ? oneweekdate.getDate() : '0' + oneweekdate.getDate();
        $("#begin_time").val(year1 + "-" + month1 + "-" + day1);
    });
    //近1个月
    $("#onemonth").click(function () {
        $("#begin_time").val(getPreMonthDay(nowdatestr, 1));
    });
    //近3个月
    $("#threemonth").click(function () {
        $("#begin_time").val(getPreMonthDay(nowdatestr, 3));
    });

    //获取当前日期前N个月的日期
    function getPreMonthDay(date, monthNum) {
        var dateArr = date.split('-');
        console.log(dateArr)
        var year = dateArr[0]; //获取当前日期的年份
        var month = dateArr[1]; //获取当前日期的月份
        var day = dateArr[2]; //获取当前日期的日
        var days = new Date(year, month, 0);
        days = days.getDate(); //获取当前日期中月的天数
        var year2 = year;
        var month2 = parseInt(month) - monthNum;
        if (month2 <= 0) {
            year2 = parseInt(year2) - parseInt(month2 / 12 == 0 ? 1 : parseInt(month2) / 12);
            month2 = 12 - (Math.abs(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;
    }


  //重写toLocaleString把时间戳转换为想要的格式
    Date.prototype.toLocaleString = function () {
        return this.getFullYear().toString() + "-" + (this.getMonth() + 1).toString() + "-" + this.getDate().toString() + "   " + this.getHours().toString() + ":" + this.getMinutes().toString() + ":" + this.getSeconds().toString();
    };