<script language="JavaScript">
<!--
function diff() {
document.form1.c3.value= '';
for (var i=1; i<6; i++){
if (document.form1["txtiddatiditta"+i].checked)
document.form1.c3.value += ' '+document.form1["txtiddatiditta"+i].value; } }
//-->

</script>

<form name="form1">
<INPUT type=checkbox value=1111 name=txtiddatiditta1 onclick="diff();">
<INPUT type=checkbox value=2222 name=txtiddatiditta2 onclick="diff();">
<INPUT type=checkbox value=3333 name=txtiddatiditta3 onclick="diff();">
<INPUT type=checkbox value=4444 name=txtiddatiditta4 onclick="diff();">
<INPUT type=checkbox value=5555 name=txtiddatiditta5 onclick="diff();">


<input type="text" name="c3" size="50"readonly>
</form>

ciao