Slightly change it back to the previous implementation, as the naive implementation does not work well when neither UTC nor local time is used. It is difficult to address this problem appropriately on Zope 2.8, as DateTime does not support pytz in this version, thus it cannot interpret arbitrary timezone names.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@39462 20353a03-c40f-0410-a6d1-a30d3c3de9de