Hi yaap,
You can not use two typo plugins on the same field.
One way to do that is to create a copy of your calendar field in your "content" or "intro" view. After that, you can apply a "date" typo plugin on the copy and apply the result in the html code like that $cck->getTypo('name_of_copied_field').
The copy of a field is a seblod field which has the same storage.
Regards,
Mehdi.