Forms Data Controls :: Add Leading Zero In Formview Edititemtemplate?

I have a formview which binds to a table where one of the fields is an integer zip-code field. I want to pad the zip with a leading zero in the edit template when the zip is something like "02134". How do I edit the bound data without "breaking" the two-way binding?

I wrote simple function to add the zero and return a string:

Public
Function padZIP(ByVal zip
As
Integer)
As
String
Dim tmpStr
As
String =
CType(zip,
String)
If tmpStr.Length = 4
Then tmpStr =
"0" & tmpStr
Return tmpStr
End
Function

and I can call it in the itemtemplate fine:

<asp:Label
ID="ZipLabel"
runat="server"
Text='<%# padZIP(container.dataitem("Zip")) %>'
Width="30"
/>

but, trying to do that in the edititemtemplate breaks the binding?