Ok I found a workaround (I get the date from a query and not from field from salesforce.com

For those who want to make Date difference (and then date shifting with the dateadd from sforce.util) here is  some code
Devangel and Benjibasik you can take it and add it to sforce.util if you want (I think it is quite close to date add and shoul integrate easily):smileyhappy:

function DateDiff(p_Interval,p_Date_1, p_Date_2){
    if(!Sforce.Util.IsDate(p_Date_1)){return "invalid date: '" + p_Date_1 + "'";}
    if(!Sforce.Util.IsDate(p_Date_2)){return "invalid date: '" + p_Date_2 + "'";}
    var dt1 = new Date(p_Date_1);
    var dt2 = new Date(p_Date_2);
   
    // get ms between dates (UTC) and make into "difference" date
    var iDiffMS = dt2.valueOf() - dt1.valueOf();
    var dtDiff = new Date(iDiffMS);

    // calc various diffs
    var nYears  = dt2.getUTCFullYear() - dt1.getUTCFullYear();
    var nMonths = dt2.getUTCMonth() - dt1.getUTCMonth() + (nYears!=0 ? nYears*12 : 0);
    var nQuarters = parseInt(nMonths/3);   
   
    var nMilliseconds = iDiffMS;
    var nSeconds = parseInt(iDiffMS/1000);
    var nMinutes = parseInt(nSeconds/60);
    var nHours = parseInt(nMinutes/60);
    var nDays  = parseInt(nHours/24);
    var nWeeks = parseInt(nDays/7);


    // return requested difference
    var iDiff = 0;       
    switch(p_Interval.toLowerCase()){
        case "yyyy": return nYears;
        case "q": return nQuarters;
        case "m": return nMonths;
        case "y":         // day of year
        case "d": return nDays;
        case "w": return nDays;
        //case "ww":return nWeeks;        // week of year    // <-- inaccurate
        case "h": return nHours;
        case "n": return nMinutes;
        case "s": return nSeconds;
        case "ms":return nMilliseconds;    // millisecond   
        default: return "invalid interval: '" + p_Interval + "'";
    }
}