In my languge german this works as expected.

What have you set in your "Preferences" for "Short Date Format:" (french=
"Style de date courte :") ?

Hi Christian, thanks for your reply.

I just check again, in my "Preferences" I have:

Fuseau horaire en cours / Current Time Zone : Europe/Paris
Style de date courte / Short Date Format : 01/07/13
Style de date longue / Long Date Format : 01 Juillet, 2013

for a new event for tomorrow while editing attendees I have: 07/02/2013

I just tried to change the "Short Date Format:" in my german webinterface.
It doesn't change in event editing window nor in invite window.
So I assume this is an template error.
Obviously there is one fix format string used, which displays according to the translation in the language used.
It should be the "Short Date Format" variable instead.

Workaround would be to change the translation of this fix date format string.
But I am not sure, which one that would be.

similar problem http://www.sogo.nu/bugs/view.php?id=2286

Notice that the user's time format will be used in the attendees window in release 2.2.0.

Ref: https://github.com/inverse-inc/sogo/commit/1f7994d1bf9302e02940de5fc75c0a53a9911d75