How to use TimeZoneInfo to get local time during Daylight Saving Time?

Question

I'm trying to use DateTimeOffset to convey a specific moment in time across any time zone. I can't figure out how to use TimeZoneInfo to deal with daylight saving time.

var dt = DateTime.UtcNow;
Console.WriteLine(dt.ToLocalTime());

var tz = TimeZoneInfo.FindSystemTimeZoneById("Central Standard Time");
var utcOffset = new DateTimeOffset(dt, TimeSpan.Zero);
Console.WriteLine(utcOffset.ToOffset(tz.BaseUtcOffset));

This prints out:

6/2/2010 4:37:19 PM
6/2/2010 3:37:19 PM -06:00

I am in the central time zone, and and we are currently in daylight saving time. I am trying to get the second line to read:

6/2/2010 4:37:19 PM -05:00

BaseUtcOffset apparently doesn't change based on DST.

How can I get the the right time with the proper offset value?

Answer 1

You need to get the UtcOffset from the TimeZoneInfo, then pass that to the ToOffset() method:

var dt = DateTime.UtcNow;
Console.WriteLine(dt.ToLocalTime());

var tz = TimeZoneInfo.FindSystemTimeZoneById("Central Standard Time");
var utcOffset = new DateTimeOffset(dt, TimeSpan.Zero);
Console.WriteLine(utcOffset.ToOffset(tz.GetUtcOffset(utcOffset)));

Answer 2

You can also use TimeZoneInfo.ConvertTimeFromUtc, which will allow for daylight saving time:

DateTime utc = DateTime.UtcNow;
TimeZoneInfo zone = TimeZoneInfo.FindSystemTimeZoneById("Central Standard Time");
DateTime localDateTime = TimeZoneInfo.ConvertTimeFromUtc(utc, zone);

Answer 3

Or better, if you don't want to hard code the time zone identifier:

TimeZoneInfo tzi = TimeZoneInfo.FindSystemTimeZoneById(TimeZoneInfo.Local.Id);
DateTime localDateTime = TimeZoneInfo.ConvertTimeFromUtc(DateTime.UtcNow, tzi);

Answer 4

I'm a beginner both at .NET and stackoverflow, so I could be wrong, but here goes:

Using TimeZoneInfo.ConvertTimeFromUtc will allow for daylight saving time, and convert to the correct time according to the time zone + a possible DST offset. However - the offset itself in the resulting object will show the offset for standard time, and not take daylight saving time into account. So if you want to do a ToString on the object, you will end up with the correct time (in hours and minutes), but the wrong offset during daylight saving time, which may lead to the wrong moment in time later in the code.

If you instead use the GetUtcOffset to get the offset for a specific time, and then do a ToOffset on the DateTimeOffset object, both the hours/minutes and the offset itself will be correctly converted, and you can safely do a ToString.

string ExpectedDateTimePattern = "yyyy'-'MM'-'dd'T'HH':'mm':'ss''zzz";
string timeZoneId = "FLE Standard Time";
string dateTimestr = "2017-10-09T09:00:00+02:00";

DateTimeOffset dto = DateTimeOffset.Parse(dateTimeStr);
TimeZoneInfo zone = TimeZoneInfo.FindSystemTimeZoneById(timeZoneId);
TimeSpan offset = zone.GetUtcOffset(dto);
dto = dto.ToOffset(offset);
string localTime = dto.ToString(ExpectedDateTimePattern);

localTime will return "2017-10-09T10:00:00+03:00".

datetimeoffset timezoneinfo getutcoffset