Actually, you can update a field that is not on the form without using SQL, as long as it's in the form's recordset.
Access creates a form property for every field in its recordset, when the recordset is opened. The property has the same name as the field.
So, if a form's Record Source is "Query1" and Query1 has columns "ID", "Customer", and "DateUpdated", but DateUpdated isn't on the form, the following statement will assign the current date and time to DateUpdated:
Me.DateUpdated = Now Rick Sprague