Just FYi, when I am not logged in Firefoy shows the forum in english, your first post yesterday is shown to me as from <<on: Yesterday at 23:15:16>> and the time in the left menu under 'Welcome guest' shows 2014/09/11, 10:06:56 which is correct local time for me.
When I log in, the forum switches to german, the times of your post stay in the correct time shown, only the date format switches (Gestern um 23:15) and in the menubar no time is shown.
In my forum profil data under Design & Layout I have:
Zeitformat: (Forumstandard)
Zeitverschiebung: 0
Aktuelle Zeit des Forums 2014-09-11, 10:07
^^ perhaps Zeitverschiebung/timeshift is where you could have a look?