datetime and time handling should always have timezone information
each input or output should be able to handle timezones.
so an editor might enter a time and his local timezone must be respected before the time is stored as UTC in the DB. (not every editor is capable to calculate UTC time from local time)
so a setting for the timezone of the user is needed (User settings)
and also every time /datetime field should have its own timezone field (preset with user settings) so an editor also can enter data for other timezones.
so the input could be saved in a clean way as UTC.
for output (or visibilty checks) the server timezone could be default, but might be set in a viewhelper, typoscript or php for generating output for other locations.
maybe a special setting 'visitor' to render the time depending on visitors location (if that information is available)