Not totally sure I follow you here. You are running a Query and for the A.D. field you are using the Alias [Birth day of the pupil]. This may make for a long line in the Query, but aren't you then spitting the Results out on either a Form or Report? If so, then you can make it short there or just create some shorter name Alias to begin with?
You would be better served using short, descriptive names in the datasource, reserving the drawn out long description for your report or form. Even if you're using just the query for export, I still like to make reports for this type of thing so I can keep it clean and simple.