function getDaysInMonth(aDate){
   // returns the last day of a given month
    var m = new Number(aDate.getMonth());
    var y = new Number(aDate.getYear());

    var tmpDate = new Date(y, m, 28);
    var checkMonth = tmpDate.getMonth();
    var lastDay = 27;

    while(lastDay <= 31){
        temp = tmpDate.setDate(lastDay + 1);
        if(checkMonth != tmpDate.getMonth())
            break;
        lastDay++
    }
    return lastDay;
}

function dateFormat(aDate, displayPat){
    /********************************************************
    *   Valid Masks:
    *   !mmmm = Long month (eg. January)
    *   !mmm = Short month (eg. Jan)
    *   !mm = Numeric date (eg. 07)
    *   !m = Numeric date (eg. 7)
    *   !dddd = Long day (eg. Monday)
    *   !ddd = Short day (eg. Mon)
    *   !dd = Numeric day (eg. 07)
    *   !d = Numeric day (eg. 7)
    *   !yyyy = Year (eg. 1999)
    *   !yy = Year (eg. 99)
   ********************************************************/

    intMonth = aDate.getMonth();
    intDate = aDate.getDate();
    intDay = aDate.getDay();
    intYear = aDate.getFullYear();

    var months_long =  new Array ('January','February','March','April',
       'May','June','July','August','September','October','November',
       'December')
    var months_short = new Array('Jan','Feb','Mar','Apr','May','Jun',
       'Jul','Aug','Sep','Oct','Nov','Dec')
    var days_long = new Array('Sunday','Monday','Tuesday','Wednesday',
       'Thursday','Friday','Saturday')
    var days_short = new Array('Sun','Mon','Tue','Wed','Thu','Fri',
        'Sat')

    var mmmm = months_long[intMonth]
    var mmm = months_short[intMonth]
    var mm = intMonth < 9?'0'+ (1 + intMonth) + '':(1+intMonth)+'';
    var m = 1+intMonth+'';
    var dddd = days_long[intDay];
    var ddd = days_short[intDay];
    var dd = intDate<10?'0'+intDate+'':intDate+'';
    var d = intDate+'';
    var yyyy = intYear;

    century = 0;
    while((intYear-century)>=100)
        century = century + 100;

    var yy = intYear - century
    if(yy<10)
        yy = '0' + yy + '';

    displayDate = new String(displayPat);

    displayDate = displayDate.replace(/!mmmm/i,mmmm);
    displayDate = displayDate.replace(/!mmm/i,mmm);
    displayDate = displayDate.replace(/!mm/i,mm);
    displayDate = displayDate.replace(/!m/i,m);
    displayDate = displayDate.replace(/!dddd/i,dddd);
    displayDate = displayDate.replace(/!ddd/i,ddd);
    displayDate = displayDate.replace(/!dd/i,dd);
    displayDate = displayDate.replace(/!d/i,d);
    displayDate = displayDate.replace(/!yyyy/i,yyyy);
    displayDate = displayDate.replace(/!yy/i,yy);

    return displayDate;
}

// demo
alert(dateFormat(new Date() , "!yyyy !dd !mmm"))