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Convert somebody's local time to the UTC time

Answer 1

+5 A:

You can't ask the framework about a particular country - but you can ask about a particular time zone.

TimeZoneInfo does take DST into account. (Heck, it wouldn't have the IsDaylightSavingTime method otherwise.) If you've got two users, one of whom is currently observing DST and the other of whom isn't, then they aren't in the same time zone.

If you could specify which locations you're talking about, I could try to find out which time zones are involved. (It's generally easier to find out the Olson names, but it shouldn't be impossible to find out the Windows IDs.)

Jon Skeet 2010-08-05 07:40:01

Ok, thanks. But are you saying that users have to change their timezone when the enter the DST? for example, normally i'm in GMT+1 Amsterdam, but then the the DST is entered, am i then in GMT + 2? Or am i in GMT + 1 in DST? If the former case, the user has to change his timezone in my app. (in that case it will work as you say, because the user which lives in a DST country does change his timezone to +2, and the user which lives in a non-DST country keeps his timezone at +1)

Michel 2010-08-05 07:44:19

@Michel: No, the time zone describes when it enters and exits DST. For example, I'm in the time zone with the Olson name "Europe/London" - that's UTC for half the year, and UTC+1 for half the year. Your user in a DST country *doesn't* change time zone - their time zone changes offset from UTC. It's a very important distinction.

Jon Skeet 2010-08-05 07:45:38

@Jon: ok, didn't know that the TimeZone has the knowledge of when the DST starts end ends. But: then also have to know where you live, because i can check the TimeZone if your zone is in DST right now, but i also have to know if your country (as there are multiple countries in your timezone) works with DST or not?

Michel 2010-08-05 07:55:55

@Jon: There is also the problem that when you change DST to normal time, you have multiple 02.15 times. So when you say: it happened in my house at 02.15, i have to ask you: "Was it the first 02.15 or the second 02.15", to determine the right UTC time.

Michel 2010-08-05 07:57:39

@Michel: No, you don't need to know the country. If your country doesn't observe DST, then you're not in the same time zone as a country which *does* observe DST. You just need to know the time zone. (And be aware that many countries operate in multiple time zones.) And yes, a given local time may have 0, 1 or 2 corresponding UTC times. It's just a natural consequence of the mess that humans have made of calendaring :( `TimeZoneInfo` has methods to handle this - `IsAmbiguousTime` and `GetAmbiguousTimeOffsets`.

Jon Skeet 2010-08-05 08:15:18

Let's say that USA uses DST and Canada is not. Then those 2 counties could not be in the same time zone ? And yet they are one "above" (on the north) the other. Strange.

Petar Repac 2010-08-05 08:52:03

@Jon: Thanks very much for your help. I've always seen the multiple entries for +1, but never thought it was for determing if you are in a DST zone or not. And the IsAmbiguousTime is a big helper.Thanks again.

Michel 2010-08-05 08:52:06

@Petar: There are *several* time zones in the US, not just one. And yes, there can be different time zones for the same longitude.

Jon Skeet 2010-08-05 08:55:00

Answer 2

+2 A:

TimeZone class has a method IsDaylightSavingTime that take a date as parameter and return a boolean indicating if that date in in daylightsavingtime for that timezone.

Andrea Parodi 2010-08-05 07:42:20

Thanks, but that only works for local times, in my case, in an asp.net server app, that will be the time of the location where the server is located. In that case i can check if the SERVER is in dst, but not if the user in a particular time zone is.

Michel 2010-08-05 07:59:20

Answer 3

+1 A:

I promote my comment to an answer. It appears to be a complicate problem. Look at http://www.timeanddate.com/time/dst2010.html, there is a table of DST in various country of the world. You cannot get this with static code because DST change not only from country to country, but also from year to year.

I guess you have to maintain a table with DST information and get information from it.

Or you can query a webservice to get such infos.

Look at http://www.earthtools.org/webservices.htm#timezone for a webservice that you can query to get time in various country that take in account DST (it work only for Western Europe time zones). Also look at http://www.geonames.org/export/web-services.html. You have to get your users geolocation to use these services.

Andrea Parodi 2010-08-05 08:38:06

I've found this: 'timeZone.GetAdjustmentRules()[0].DaylightTransitionStart' which tells me when the dst starts in a particular zone, is that the same?

Michel 2010-08-05 08:53:54

I look in MSDN. It's about the same, but according to info on http://www.timeanddate.com/time/dst2010.html DST information is a dynamic one, that change in time according to countries laws modification, so it cannot be "embedded" in a static framework. Anyway, I guess modification in laws doesn't happen so frequently, so if you don't bother on a mistake in the calculation once in a while, you can go with DaylightTransitionStart

Andrea Parodi 2010-08-05 09:06:58

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Convert somebody's local time to the UTC time

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