Also worth noting: there are many time zone variations beyond DST, and some are defined in terms of partial hours (30 or 15 minute variations).

True story: when I was at ITA Software, Orbitz ran hundreds of instances of QPX, our low fare search software, on their own servers. We had an ops fire drill one weekend because customers were complaining that the site was showing incorrect prices. The root cause? A single machine in their server farm had the time wrong, so advance purchase was computed incorrectly for any query sent to that particular machine. That was fun to debug.

Even the meaning of, say, a minium stay requirement is hard to precisely define. If you fly across the international date line and back on a Friday night, did you have a Saturday stay or not? There are so many flights going so many places, questions like this actually come up in practice.