.net javascript masterpage conflict

[janetb]

I'm not a Javascript person, so please be kind. I've got an asp.net 2 page that has a master page. I've got a javascript that works fine in a regular asp.net page within the same web. Move it to the page with the master page, and it won't work. Any help appreciated. Also, what's the difference between function setDifference(form) and setDifference = function(form)? Here's the basic stuff:

<script>
setDifference = function(form)
{
var x = document.getElementById('dtTxt1').value;
var y = document.getElementById('dtTxt2').value;
var dt1 = Date.parse(x)
var dt2 = Date.parse(y)
document.getElementById('resultTxt').value = (dt1.valueOf() - dt2.valueOf()) / (60 * 60 * 24 * 1000)
}
</script> 

<asp:TextBox ID="dtTxt1" name="dtTxt1" runat="server" onkeyup="BLOCKED SCRIPTsetDifference(this.form);" onkeydown="BLOCKED SCRIPTsetDifference(this.form);" /><br />
<asp:TextBox name="dtTxt2" id="dtTxt2" runat="server" onkeyup="BLOCKED SCRIPTsetDifference(this.form);" onkeydown="BLOCKED SCRIPTsetDifference(this.form);" />
<br /><asp:textbox name="resultTxt" id="resultTxt" runat="server" /><br /><br />