/*格式 getBeforeDate('2015,5,20') */

/*

* auth:120975587@qq.com

* time:2015.5.6 9:45

* ******************

*/

function getBeforeDate(n){

    var now = new Date();

    var aftertime = new Date(n);

    var year = now.getFullYear();

    var mon= now.getMonth()+1;

    var day= now.getDate();

    var year_after = aftertime.getFullYear();

    var mon_after = aftertime.getMonth()+1;

    var day_after = aftertime.getDate();

    var chs = 0;

    //获取当月的天数

    function DayNumOfMonth(Year,Month)

    {

        return 32 - new Date(Year,Month-1,32).getDate();

    }

    if(aftertime.getTime() - now.getTime() < 0){

        var temp1 = day_after;

        var temp2 = mon_after;

        var temp3 = year_after;

        day_after = day;

        mon_after = mon;

        year_after = year;

        day = temp1;

        mon = temp2;

        year = temp3;

    }

    if(year == year_after){//不跨年

        if(mon == mon_after){//不跨年不跨月

            chs += day_after-day;

        }else{//不跨年跨月

            chs += DayNumOfMonth(year,mon)- day+1;//加上第一个不满的

            for(var i=1;i< mon_after-mon;i++){

                chs += DayNumOfMonth(year,mon+i);

            }

            chs += day_after-1;//加上

        }

    }else{//存在跨年

        chs += DayNumOfMonth(year,mon)- day+1;//加上开始年份不满的一个月

        for(var m=1;m<12-mon;m++){

            chs += DayNumOfMonth(year,mon+m);

        }

        for(var j=1;j < year_after-year;j++){

            if((year+j)%400 == 0 || (year+j)%4 == 0 && (year+j)%100 != 0){

                chs += 366;

            }else{

                chs += 365;

            }

        }

        for(var n=1;n <= mon_after;n++){

            chs += DayNumOfMonth(year_after,n);

        }

        chs += day_after-1;

    }

    if(aftertime.getTime() - now.getTime() < 0){

        return -chs;

    }else{

         return chs;

    }

}