How do you convert a Javascript timestamp into UTC format?

I would go with (new Date(integer)).toUTCString(),

but if you have to have the 'pm', you can format it yourself:

function utcformat(d){
    d= new Date(d);
    var tail= 'GMT', D= [d.getUTCFullYear(), d.getUTCMonth()+1, d.getUTCDate()],
    T= [d.getUTCHours(), d.getUTCMinutes(), d.getUTCSeconds()];
    if(+T[0]> 12){
        T[0]-= 12;
        tail= ' pm '+tail;
    }
    else tail= ' am '+tail;
    var i= 3;
    while(i){
        --i;
        if(D[i]<10) D[i]= '0'+D[i];
        if(T[i]<10) T[i]= '0'+T[i];
    }
    return D.join('/')+' '+T.join(':')+ tail;
}

alert(utcformat(1291656749000))

/* returned value: (String) 2010/12/06 05:32:29 pm GMT */

(new Date(1291656749000)).toUTCString()

Is this what you're looking for?