Personalised Clock is an app built with Python that runs on Google App Engine.
Users can personalise the clock's timezone by logging in with their Google ID.
Their preferences are stored in an App Engine datastore that lives in the cloud.
When the users log in, their previous time zone settings are recalled.
View a live demo (& personalise your own clock) at http://jaredtrivial.appspot.com/
See http://jaredblyth.com/page.php?id=78 for more information.