After I did a talk on Timezones, I often get asked the same Question:
why not simply convert all dateTime to UTC when saving to DB? Then display with appropriate timezone
Remember your Summer-vacation 2016 in Antalya, Turkey? Oh, come on: just for the sake of an example!
You met this other person in and you made an appointment for a christmas call for the 25th of December at 14:00H local Turkish Time. So your appointment is (in ISO, because we use ISO-Datetimes, don’t we?) 2016-12-25 14:00:00 Europe/Istanbul. At the time you made the appointment you would then convert that to UTC using the TimezoneDB that is available at that moment. That would resolve to 2016-12-25 12:00:00 UTC. At the time you made the appointment it was DaylightSavingTime (so it had an offset of 3 hours at that time) but the TimezoneDB already had the information that Turkey would move back from DST sometime in October so at the time of your appointment the offset would only be 2 hours. So everything is fine.
Now the 25th of December arrives and you take your Phone at 12:00:00 UTC and dial the number. And then it happens! No one is answering your call. What the heck?
What happened? The person you wanted to call simply wasn’t at home (or wherever the phone was or had the phone turned off) because they waited for 3 quarters of an hour for your phonecall and then simply gave up and went to wherever.
Why? Well, when you made your appointment, no one knew, that the Turkish gouvernment would decide to stay on DST even in winter. They never switched back from DST so their offset to UTC is still 3 hours at the 25th of December. So your conversion to UTC was based on the assumption that Turkey would move back from DST as they did for multiple years in a row. But as politics go, they suddenly decided to not do that any more. So you made your conversion on an assumption. Had you noted 14:00 Europe/Istambul instead of 12:00 UTC everything would have been fine, as your updated timezone-DB would have easily reminded you at the correct time (11:00 UTC) of your appointment.
YOu might think, that the timezonedb doesn’t that often change: At the time of writing this the current timezone-database you can download at the IANA is version 2016j. That’s the 10th revision of the database in 2016. and every release contains more than one change. So at least 10 changes to timezone entries alone in 2016. And those changes are not only such changes as turkeys remaining in DST (which was announced about 1 month in advance) but there are such things as Tonga moving from an offset of +13:00 to -14:00 (skipping a whole day on the way) in 2008 or a simple “Hey, we just heard that Antarctic station XYZ moved their offset by 3 hours 3 weeks ago”.
The timezone-database is “valid” for about ± 4 weeks but the further a datetime is from the release date of the current timezone-database the more insecure it becomes to convert to or from a certain timezone.
Did that make sense? Feel free to comment or ping me via email, twitter or on IRC to discuss the matters.