document.getElementById("Lbl_Timer").innerHTML = "jjj";

so try
js += "document.form[0]." + Lbl_TimeCounter.ClientID +
> ".innerHTML = timeValue";

I don't know if you are but don't assign this value to the same label
the script is attached to !
(though it might be interesting to see if it works ... it might as the
script will be held in mem ! dunno about functions though)