Code involving dates and times is very often broken. This is partly due to poor APIs on many platforms, and partly due to a lack of awareness on the part of developers. Many developers despair when they think about time zones or different calendar systems, but there's good news: it's not really that bad. Obviously it helps if you're using a decent API (*cough* Noda Time *cough*) but even if you're forced to use a poor API, if you have a clear understanding of what you're trying to achieve, you can write code you can be confident in. In this session I'll present a minimal set of concepts to think about, highlight some of the gotchas to be aware of, and suggest strategies for making it easy to test your code.