18

If daylight saving time is in effect, and a date object has been saved into the database (UTC format) which you retrieve to show it in the view (for example the view in asp.net-mvc).

And you do that by using this method:

public static DateTime ConvertToLocalTimeFromUtcTime(DateTime utcDate, string timeZoneId)
{
    TimeZoneInfo localZone = TimeZoneInfo.FindSystemTimeZoneById(timeZoneId);
    DateTime localTime = TimeZoneInfo.ConvertTimeFromUtc(utcDate, localZone);

    if (localZone.IsDaylightSavingTime(localTime)) 
        localTime = localTime.AddHours(1); // is this needed !?

    return localTime;
}

The question is, does TimeZoneInfo.ConvertTimeFromUtc() handle DST's or do you have to check that yourself and either add or subtract X hour(s) to the date object?

Same question for when persisting a date object to the database by converting it to UTC format with ToUniversalTime().

Quoter
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    Your timezone is probably using DST right now, which will make your testing really easy to do ;) – Andrew Morton Apr 18 '14 at 16:25
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    Yes. Whether DST is in effect in a particular locale is a local political decision. Not one you ever know enough about on a web server. Always make the conversion in the browser, it knows. – Hans Passant Apr 18 '14 at 16:30
  • @AndrewMorton, true, and I did saw 2 hours being subtracted from the time given from the client. But how about vice versa? Are you saying that I don't have to convert the time in the zone the user is in on server side? – Quoter Apr 18 '14 at 17:01
  • Maybe this will be useful: [Convert UTC date time to local date time using JavaScript](http://stackoverflow.com/questions/6525538/convert-utc-date-time-to-local-date-time-using-javascript). – Andrew Morton Apr 18 '14 at 17:17
  • @HansPassant - Converting in the browser is not necessarily the best idea. It's one approach, but [it suffers from errors](http://codeofmatt.com/2013/06/07/javascript-date-type-is-horribly-broken/). If you have the time zone id (which the OP does in this case), then `TimeZoneInfo` is going to be more accurate than JavaScript. Besides, there are plenty of cases where you may want to convert to a *different* time zone, not just the local time zone of the browser. – Matt Johnson-Pint Apr 18 '14 at 23:37

1 Answers1

22

Yes. ConvertTimeFromUtc will automatically handle daylight saving time adjustments, as long as the time zone that you are targeting uses daylight saving time.

From the MSDN documentation:

When performing the conversion, the ConvertTimeFromUtc method applies any adjustment rules in effect in the destinationTimeZone time zone.

You should not try to add an additional hour in your conversion. That will give you an incorrect translation.

Regarding DateTime.ToUniversalTime, it does take DST into account, but be careful with this method. It assumes that the input value is in the computer's local time zone. If you just need to mark it with DateTimeKind.Utc, then use DateTime.SpecifyKind instead.

Matt Johnson-Pint
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