Risolto, mancava la riga in rosso:

codice:
<html>

<head>

<script language="javascript" type="text/javascript">
 
<!--
function ValidateForm(Qform) 
{
 
  for (var a = 0; a < Qform.elements.length; a++) 
    {
    var campo = Qform.elements[a];
 
        if (campo.value.length <= 0)
 
        {
            window.alert('KO-1.');
            campo.focus();
            return false;
          }
          
     }
          
var re = new RegExp("^([0-9]{2}) : ([0-9]{2})$", "");
 
if (!Qform.T1.value.match(re))
    {
      alert("KO-3");
      Qform.T1.focus();
      Qform.T1.select();
      return(false);
    }   

    var T = Qform.T1.value;   

    T = T.split(/\D+/);
    
    var D1 = new Date();
    D1 = new Date(D1.setHours(T[0]*1,T[1]*1,0,0));
    
    if(!D1 || D1 < new Date()) 
    
    
    {
      alert("KO-2");
      Qform.T1.focus();
      Qform.T1.select();
      return(false);      
    }  

             
return(true);
}
 
//-->
</script>
</head>

<body>

<form action="other.htm" method="post" name="Qform" onsubmit="return(ValidateForm(this));">

	

<input type="text" name="T1" size="20"></p>
	

<input type="submit" value="GO" name="B1"></p>
</form>

</body>

</html>