var show_time=false;
  var timerID=null;

  function stop(){
     if (show_time){
         clearTimeout(timerID);
         document.getElementById("date_time").value=" ";
     }
     show_time=false;
  }

  function start(form){
     var today=new Date();
     var display_value ="Dnes je " + today.getDate() + ". "  + (today.getMonth()+1) + ". " + (today.getFullYear());
     display_value += "   " + today.getHours()
     if(today.getMinutes() < 10){
        display_value+=":0" + today.getMinutes();
     }
     else{
        display_value+=":" + today.getMinutes();
     }
     if (today.getSeconds() < 10){
        display_value+=":0" + today.getSeconds();
     }
     else{
        display_value+=":" + today.getSeconds();
     }
     if(today.getHours()>=12){
        display_value+="  "
     }
     else{
        display_value+="  "
     }

        document.getElementById("date_time").value=display_value;
        timerID=setTimeout("start()",100);
        show_time=true;
  }