C# – How to work with time zones in ASP.NET


I am working on an "online reminder system" project (ASP.NET 2.0 (C#) / SQL Server 2005)

As this is a reminder service which will send the mail to users on a particular dates. But the problem is users are not from a specific countries, they are from all over the world and from different time zones. Now When I am registering I am asking for users time zone in the same way as windows asks our time zone at the time of installation.

But I am not getting the if the user selected (+5.30) or something timezone then how to handle this time zone in my asp.net application. How to work according to timezone.

And please suggest if there is any better way to handle timezones in this application ??


Best Solution

First thing is to make sure which time zone your data is in. I would recommend making sure that any DateTime that you store, is stored in UTC time (use the DateTime.ToUniversalTime() to get hold of it).

When you are to store a reminder for a user, you will need the current UTC time, add or remove the user's time zone difference, and convert that new time back to UTC; this is what you want to store in the DB.

Then, when you want to check for reminders to send, you simply need to look in the database for reminders to send out now, according to UTC time; essentially get all reminders that have a time stamp that is before DateTime.Now.ToUniversalTime().

Update with some implementation specifics: You can get a list of time zones from the TimeZoneInfo.GetSystemTimeZones() method; you can use those to show a list of time zones for the user. If you store the Id property from the selected time zone, you can create a TimeZoneInfo class instance from it, and calculate the UTC time for a given local date/time value:

TimeZoneInfo tzi = TimeZoneInfo.FindSystemTimeZoneById("<the time zone id>");
// May 7, 08:04:00
DateTime userDateTime = new DateTime(2009, 5, 7, 8, 4, 0);
DateTime utcDateTime = userDateTime.Subtract(tzi.BaseUtcOffset);