One notable addition: relative time is the same for everyone.

What I mean by this is that instead of messing with timezones (by trying to guess the user's timezone or even worse, asking for it) in most cases it is sufficient to tell the user something has happened x hours ago, or y days ago.

If you're programming in PHP, I can recommend this book: http://www.amazon.com/architects-Guide-Date-Time-Programming...