To open the form as a hidden form, place code in the Load event of your
startup form similar to:

DoCmd.OpenForm "MyTimerForm",,,,,acHidden

As far as creating the form goes, any form that remain open all the time
will work. It doesn't have to be a hidden form. The advantage of a hidden
form is that your user isn't likely to try and close it. The form wouldn't
need any controls on it, just the code in the Timer event.

Sample Code:
If DCount("[DateField]", "[MyTable]", "[DateField]<=" & Date) > 0 Then
   MsgBox "There are records that need tending too!", vbOkOnly +
vbInformation, "Overdue Items"
End If