yyyy-mm-dd
Depending on what other factors are taken into account when sorting it may be that this fails to put the tracks of any given single/album in the correct order so you may need to add the track number into the value. E.g.
yyyy-mm-dd.tt
This scheme won't be able to fix the general ordering in iTunes, but it would let manage a custom sort order in a playlist of all your Glee tracks.
tt2