Hi Mahlon,
The intention for these Merge fields is that they would be based off your Display Preferences (Under Preferences > Display for others reading).
However, it appears this is not functioning as intended at the moment. After some testing, Ive discovered I can't affect the Email Date Merge fields from dsplaying as anything other than DD MMMM YYYY.
Ive logged a ticket for the team to look into this one, and will report back once its been resolved.
Thanks Mahlon.
-Ben