First make your datetime field nullable.
Then you will need some logic in your asp page:

<%

cmdText = " Select * from sometable "
Set rs = CreateObject("ADODB.Recordset")
With rs
.ActiveConnection = cn
.CursorType = 2
.LockType = 3
.Open cmdText
End With

' If statement to handle blank form value
if request("SomeDateColumn") <> "" then
rs("SomeDateColumn") = request("SomeDateColumn")
else
rs("SomeDateColumn") = null
end if


rs.Update

%>



Jeff