c# - Does ConvertTimeFromUtc() and ToUniversalTime() handle DST?

Question

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().

Answer 1

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.