javascript的日期加减

 function TimeCom( dateValue )
    {
        var newCom = new Date( dateValue );
        this.year = newCom.getYear();
        this.month = newCom.getMonth()+1;
        this.day = newCom.getDate();
        this.hour = newCom.getHours();
        this.minute = newCom.getMinutes();
        this.second = newCom.getSeconds();
        this.msecond = newCom.getMilliseconds();
        this.week = newCom.getDay();
    }

    function DateDiff(interval,date1,date2)
    {
        var TimeCom1 = new TimeCom(date1);
        var TimeCom2 = new TimeCom(date2);
        var result;
        switch(String(interval).toLowerCase())
        {
            case "y":
            case "year":
            result = TimeCom1.year-TimeCom2.year;
            break;
            case "n":
            case "month":
            result = (TimeCom1.year-TimeCom2.year)*12+(TimeCom1.month-TimeCom2.month);
            break;
            case "d":
            case "day":
            result = Math.round((Date.UTC(TimeCom1.year,TimeCom1.month-1,TimeCom1.day)-Date.UTC(TimeCom2.year,TimeCom2.month-1,TimeCom2.day))/(1000*60*60*24));
            break;
            case "h":
            case "hour":
            result = Math.round((Date.UTC(TimeCom1.year,TimeCom1.month-1,TimeCom1.day,TimeCom1.hour)-Date.UTC(TimeCom2.year,TimeCom2.month-1,TimeCom2.day,TimeCom2.hour))/(1000*60*60));
            break;
            case "m":
            case "minute":
            result = Math.round((Date.UTC(TimeCom1.year,TimeCom1.month-1,TimeCom1.day,TimeCom1.hour,TimeCom1.minute)-Date.UTC(TimeCom2.year,TimeCom2.month-1,TimeCom2.day,TimeCom2.hour,TimeCom2.minute))/(1000*60));
            break;
            case "s":
            case "second":
            result = Math.round((Date.UTC(TimeCom1.year,TimeCom1.month-1,TimeCom1.day,TimeCom1.hour,TimeCom1.minute,TimeCom1.second)-Date.UTC(TimeCom2.year,TimeCom2.month-1,TimeCom2.day,TimeCom2.hour,TimeCom2.minute,TimeCom2.second))/1000);
            break;
            case "ms":
            case "msecond":
            result = Date.UTC(TimeCom1.year,TimeCom1.month-1,TimeCom1.day,TimeCom1.hour,TimeCom1.minute,TimeCom1.second,TimeCom1.msecond)-Date.UTC(TimeCom2.year,TimeCom2.month-1,TimeCom2.day,TimeCom2.hour,TimeCom2.minute,TimeCom2.second,TimeCom1.msecond);
            break;
            case "w":
            case "week":
            result = Math.round((Date.UTC(TimeCom1.year,TimeCom1.month-1,TimeCom1.day)-Date.UTC(TimeCom2.year,TimeCom2.month-1,TimeCom2.day))/(1000*60*60*24)) % 7;
            break;
            default:
            result = "invalid";
        }
        return(result);
    }

    function DateAdd(interval, num, dateValue)
    {
        var newCom = new TimeCom(dateValue);
        switch(String(interval).toLowerCase())
        {
            case "y": case "year": newCom.year += num; break;
            case "n": case "month": newCom.month += num; break;
            case "d": case "day": newCom.day += num; break;
            case "h": case "hour": newCom.hour += num; break;
            case "m": case "minute": newCom.minute += num; break;
            case "s": case "second": newCom.second += num; break;
            case "ms": case "msecond": newCom.msecond += num; break;
            case "w": case "week": newCom.day += num*7; break;
            default: return("invalid");
        }
        var now = newCom.year+"/"+newCom.month+"/"+newCom.day+" "+newCom.hour+":"+newCom.minute+":"+newCom.second;
        return(new Date(now));
    }

2.另一个简单例子
 日期减去天数等于第二个日期
<script language=Javascript>
function cc(dd,dadd)
{
//可以加上错误处理
var a = new Date(dd)
a = a.valueOf()
a = a - dadd * 24 * 60 * 60 * 1000
a = new Date(a)
alert(a.getFullYear() + "年" + (a.getMonth() + 1) + "月" + a.getDate() + "日")
}
cc("12/23/2002",2)
</script>

这里不得不做补充,浪费好多时间得出教训:
Javascript 对时间的代号
0-11数字表示1-12月:  var a= new Date(2006,5,6)  结果是2006-6-6
0-6表示星期
1-31表示日期
0-23小时
0-59分钟,秒