Let’s You and Him Fight

Time zones in Google App Engine

Time zones are a human — by which I mean comical and insanely complex — thing. If you’re the sort of person who likes to bore people with inane trivia, and I know I am, you’ll find endless grist for your mill in Riyadh Solar Time, where they tried to make 0:00 fall on sunset. Every day. In the best case, that meant the DST offset changed daily. In the worst case, it changed moment by moment. Imagine the hassle of keeping your clocks up to date.

But even in the relatively sane time zone world we have here in the US, things are still pretty complex. There are places that don’t observe DST, and places where time zone borders make things complicated. So my policy for time zone handling in software is similar to my policy for unicode handling in software: convert to UTC on input to the application, and convert to the desired timezone on output. That way, you can do all the datetime math you need, without worrying about getting caught in a time rift or something.

Another way datetimes are similar to strings is this: a datetime without a timezone attached is meaningless. If I tell you a meeting is at 14:00, you could perhaps guess the time zone based on where the meeting will be held. But if you couldn’t do that (perhaps it is an online meeting) you actually don’t know when it will occur at all, because that might be 14:00 Central Time, 14:00 Pacific Time, or 14:00 Central European Time.

Google’s App Engine drops the ball a little when it comes to datetimes. If fed a timezone-aware datetime, it will convert it to UTC for storage. But when the value is read back out of the datastore, there will be no timezone attached. The context of the datetimes is lost, and you could make a case that part of the value of the datetimes is being lost as well.

So I wrote a type wrapper that fixes that. It attaches the UTC timezone to datetimes when they’re read out, as well as ensuring that only UTC datetimes are stored and everything else is converted. That part is perhaps a little redundant, but I wanted consistency.