How to elegantly deal with timezones

Question

I have a website that is hosted in a different timezone than the users using the application. In addition to this, users can have a specific timezone. I was wondering how ot

Answer 1

In the events section on sf4answers, users enter an address for an event, as well as a start date and an optional end date. These times are translated into a datetimeoffset in SQL server that accounts for the offset from UTC.

This is the same problem you are facing (although you are taking a different approach to it, in that you are using DateTime.UtcNow); you have a location and you need to translate a time from one timezone to another.

There are two main things I did which worked for me. First, use DateTimeOffset structure, always. It accounts for offset from UTC and if you can get that information from your client, it makes your life a little easier.

Second, when performing the translations, assuming you know the location/time zone that the client is in, you can use the public info time zone database to translate a time from UTC to another time zone (or triangulate, if you will, between two time zones). The great thing about the tz database (sometimes referred to as the Olson database) is that it accounts for the changes in time zones throughout history; getting an offset is a function of the date that you want to get the offset on (just look at the Energy Policy Act of 2005 which changed the dates when daylight savings time goes into effect in the US).

With the database in hand, you can use the ZoneInfo (tz database / Olson database) .NET API. Note that there isn't a binary distribution, you'll have to download the latest version and compile it yourself.

At the time of this writing, it currently parses all of the files in the latest data distribution (I actually ran it against the ftp://elsie.nci.nih.gov/pub/tzdata2011k.tar.gz file on September 25, 2011; in March 2017, you'd get it via https://iana.org/time-zones or from ftp://fpt.iana.org/tz/releases/tzdata2017a.tar.gz).

So on sf4answers, after getting the address, it is geocoded into a latitude/longitude combination and then sent to a third-party web service to get a timezone which corresponds to an entry in the tz database. From there, the start and end times are converted into DateTimeOffset instances with the proper UTC offset and then stored in the database.

As for dealing with it on SO and websites, it depends on the audience and what you are trying to display. If you notice, most social websites (and SO, and the events section on sf4answers) display events in relative time, or, if an absolute value is used, it's usually UTC.

However, if your audience expects local times, then using DateTimeOffset along with an extension method that takes the time zone to convert to would be just fine; the SQL data type datetimeoffset would translate to the .NET DateTimeOffset which you can then get the universal time for using the GetUniversalTime method. From there, you simply use the methods on the ZoneInfo class to convert from UTC to local time (you'll have to do a little work to get it into a DateTimeOffset, but it's simple enough to do).

Where to do the transformation? That's a cost you are going to have to pay somewhere, and there's no "best" way. I'd opt for the view though, with the timezone offset as part of the view model presented to the view. That way, if the requirements for the view change, you don't have to change your view model to accommodate the change. Your JsonResult would simply contain a model with the IEnumerable and the offset.

On the input side, using a model binder? I'd say absolutely no way. You can't guarantee that all the dates (now or in the future) will have to be transformed in this way, it should be an explicit function of your controller to perform this action. Again, if the requirements change, you don't have to tweak one or many ModelBinder instances to adjust your business logic; and it is business logic, which means it should be in the controller.