Billy has quite a good grasp of the idea of world time zones. He knows that different parts of the world are at different times and can tell you facts like when it's midnight in London and America (I'm not sure which part of America). We've never done a "lesson" on time zones - it's come about mainly due to him "needing" to know. Partly its due to sites such as Club Penguin and Webkinz. Since they are American sites, the competitions etc close at American times, so for us Aussies, we have to translate. So there's been lots of discussions & demonstrations about that as he initially tried to wrap his mind around it.
We have friends in London & America, so it makes it far more real to him to be able to say things like "It's night time in London, D will be in bed now; A will just be getting up now" etc.
Sites like these have been handy as well:
At the observatory we also got to watch mechanised models of the earth, moon & sun, so that also added to his understanding of it all.
Relevant & useful - natural learning at its finest.