You’ve built a prototype, everything is going great. All your dates and times look great, they load and store correctly, everything is spiffy. You have your buddy give it a whirl, and it works great for them too. Then you have a friend in Curaçao test it, and they complain that all the times are wrong – time zones strike again!
But, you’ve got this covered. You just add an offset to every stored date/time, so you know the origin time zone, and then you get the user’s time zone, and voila! You can correct for time zones! Everything is going great, summer turns to fall, the leaves change, the clocks change, and it all falls apart again. Now you’re storing dates in various time zones, without DST information, you’re adjusting them to the user’s time zone, trying to account for DST, trying to find a spot here or there where you forgot to account for offsets…
Don’t fall into this trap. UTC is always the answer. It is effectively time-zone-less, as it has an offset of zero and does not observe daylight savings time. It’s reliable, it’s universal, it’s always there when you need it, and you can always convert it to any time you need. Storing a date/time with time zone information is like telling someone your age by giving your birthday and today’s date – you’re dealing with additional data and additional processing with zero benefit.
When starting a project, you’re going to be better off storing all dates as UTC from the get-go; it’ll save you innumerable headaches later on. I think it is atrocious that .NET defaults to system-local time for dates; one of the few areas where I think Java has a clearly better design. .NET’s date handling in general is a mess, but simply defaulting to local time when you call DateTime.Now encourages developers to exercise bad practices; the exact opposite of the stated goals of the platform, which is to make sure that the easy thing and the correct thing are, in fact, the same thing.
On a vaguely related note, I’ve found a (in my opinion) rather elegant solution for providing localized date/time data on a website, and it’s all wrapped up in a tiny Gist for your use: https://gist.github.com/aprice/7846212
This simple jQuery script goes through elements with a data attribute providing a timestamp in UTC, and replaces the contents (which can be the formatted date in UTC, as a placeholder) with the date/time information in the user’s local time zone and localized date/time format. You don’t have to ask the user their time zone or date format.
Unfortunately it looks like most browsers don’t take into account customized date/time formatting settings; for example, on my computer, I have the date format as yyyy-mm-dd, but Chrome still renders the standard US format of mm/dd/YYYY. However, I think this is a relatively small downside, especially considering that getting around this requires allowing users to customize the date format, complete with UI and storage mechanism for doing so.