Can be duplicated on tikiwiki.org
start and end time in tiki-calendar.php (add or edit mode) takes the GMT time, even if the current user is in local time.
This is not such a problem when adding an event because the user is expected to choose the time.
However, when someone edits an existing event, the start & end times are reverted back to GMT. When the user saves the event, the time is several hours off.
Also affects tiki-edit_article.php, tiki-edit_submission.php and tiki-view_tracker.php (when submitting a tracker)
To duplicate the bug, you must be in a different timezone than your server.