Based on your information it's most likely javascript on the planner which picks your local timezone (it's checked/calculated on the client - your PC).
To change it, go to Date/Time properties (or simply double click on the clock - see here for more details http://windows.microsoft.com/en-us/windows/set-clock#1TC=windows-7) and change your timezone to desired one (German in your case). You need to restart your browser to pick up the change.
As changing timezone permanently(or switching back and forth) may be inconvenient, you probably could create another user with your desired settings and then just run your preferred browser in the context of that user (shift-right click and select 'run as different user'). Disclaimer: I have not tested it.
Additional note: Windows can show up to 3 clocks which you may find useful (information on how to setup in the linked article)