[BUGFIX] Datemenu should use getDateField()

Change-Id: Id60271a9abc76dd2649f8cd2193ed831d915809a
Resolves: #55898
Reviewed-on: https://review.typo3.org/27568
Reviewed-on: https://review.typo3.org/27569
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer