Using the DateTime class
Good article on MSDN on using the DateTime class, and how not to use it. It's really easy to not consider some of the subtle issues.
In almost every program I've ever written, if a date calculation went wrong during a time zone switch, well, nothing horribly bad would happen, so I usually tend to ignore this stuff - but it's good to at least know what you need to know when you do need to do it right.