I don't know if it is Zopes problem, I don't know if it is Plones problem, but I do know that it was my problem. The problem description was simple: When entering events in plone, the time always jumped back about 10 or 12 hours. Editing these events made it jump back further...
The problem at the end was related to the way the functions in DateTime.py interpret the EST returned as the timezone. For example, this is the output of date(1):
[~] edwin@k7>TZ=Australia/Sydney date Sun Jul 30 19:33:36 EST 2006 [~] edwin@k7>TZ=EST date Sun Jul 30 04:33:30 EST 2006
So despite that the date function returns the Australian EST, the timezone EST reverts back to the US version.
And now the gory details on how to resolve this: Search for the file DateTime.py (/usr/local/www/Zope/lib/python/DateTime/DateTime.py on my machine), search for the definition of the array _zmap and in there, search of the keypair 'est':'US/Eastern'. Replace the est at the beginning to something else (I made it eest), then restart the Zope daemon et voila.