var controllo=0;
function valida(){
  if (document.theform.data_partenza.value==''){
    alert ("Datum obligatorisch");
    return false;
    controllo=1;
  }

  if (document.theform.data_arrivo.value==''){
    alert ("Datum obligatorisch");
    return false;
    controllo=1;
  }
  if (document.theform.data_partenza.value==document.theform.data_arrivo.value){
    alert ("Ungueltiger Zeitraum!");
    return false;
    controllo=1;
  }
if(controllo==0)
document.theform.pulsante.enabled=false;
document.theform.submit();
}