js 日期按年月日加减

<script>
function isleapyear(year)
{
    if(parseInt(year)%4==0 && parseInt(year)%100!=0)return true;
    if(parseInt(year)%400==0)return true;
    return false;
}

function datedrift(date,difference)
{
    if(date.constructor!==Date)return Date();
    var year=parseInt(date.getUTCFullYear()),
        mon=parseInt(date.getUTCMonth())+1,
        day=parseInt(date.getUTCDate());
    if(typeof difference!="string" || difference.length==0)return date;
    var units=difference.match(/([+-]\d{1,3}[myd])/ig);
    for(var i=0;i<units.length;i++)
    {
        if(units[i].charAt(0)=='+')
        {
            var mark=units[i].substr(-1,1);
            var value=parseInt(units[i].substr(1,units[i].length-2));
            if(mark =='y')
            {
                year=year+value;
                if(day==29)
                {
                    year=year+value;
                    if(mon==2)
                    {
                        if(!isleapyear(year))
                            day=28;
                    }
                }
            }
            else if(mark == 'm')
            {
                year+=Math.floor((value+mon-1)/12);
                mon=(value+mon-1)%12+1;
                if(day==29 && mon==2 && !isleapyear(year))day=28;
                else if(day>=30 && mon==2)
                {
                    if(isleapyear(year))day=29;
                    else day=28;
                }
                else if(day==31 && (mon==4 || mon==6 || mon==9 || mon==11))day=30;
            }
            else if(mark== 'd')
            {
                var tmp=new Date();
                tmp.setTime(Date.parse(year+'-'+mon+'-'+day)+value*24*36e5);
                year=tmp.getUTCFullYear();
                mon=tmp.getUTCMonth()+1;
                day=tmp.getUTCDate();
            }

        }
        else
        {
            var mark=units[i].substr(-1,1);
            var value=parseInt(units[i].substr(1,units[i].length-2));
            switch(mark)
            {
                case "y":
                    year-=value;
                    if(day==29)
                    {
                        if(mon==2)
                        {
                            if(!isleapyear(year))
                                day=28;
                        }
                    }
                break;
                case "m":
                    var minusyear=Math.floor(value/12);
                    if(value%12>0)
                    {
                        year=year-minusyear-1;
                        year+=Math.floor((mon+11-value%12)/12);
                        mon=(mon+11-(value%12))%12+1;

                        if(day==29 && mon==2 && !isleapyear(year))day=28;
                        else if(day>=30 && mon==2)
                        {
                            if(isleapyear(year))day=29;
                            else day=28;
                        }
                        else if(day==31 && (mon==4 || mon==6 || mon==9 || mon==11))day=30;
                    }
                    else
                    {
                        year=year-minusyear;
                        if(day==29 && mon==2 && !isleapyear(year))day=28;
                        else if(day>=30 && mon==2)
                        {
                            if(isleapyear(year))day=29;
                            else day=28;
                        }
                        else if(day==31 && (mon==4 || mon==6 || mon==9 || mon==11))day=30;
                    }
                break;
                case "d":
                    var tmp=new Date();
                    tmp.setTime(Date.parse(year+'-'+mon+'-'+day)-value*24*36e5);
                    year=tmp.getUTCFullYear();
                    mon=tmp.getUTCMonth()+1;
                    day=tmp.getUTCDate();
                break;
            }
        }
    }
    alert([year,mon,day]);
}
var curdate=new Date();
datedrift(curdate,"+5m -5m");
</script>