The subkey expiration change dialog should have a note "Your primary key expires on: <date>" and then limit the options to dates before that date. Since it does not make sense to have subkeys that have a longer validity then the primary key. Also the context menu entry should have an Icon and be renamed to "Change expiration date". Since "End of validity period" is a bit confusing. Related to that we should change the "Valid until:" column to "Expires". And if none is set, show the expiration date of the primary key in that column for other subkeys.