C# DateTime - 将 DateTimeOffset 转换为另一个 TimeZone

Question

When converting a DateTimeOffset to another TimeZone, the OffSet is incorrect.

I've read many articles and experimented for too many hours, but can't see what I'm missing here:

// It's June in the UK and we're in British Summer Time, which is 1 hour ahead of UTC (GMT)
var UKoffsetUtc = new TimeSpan(1, 0, 0);

// It's 4pm - declare local time as a DateTimeOffset
var UKdateTimeOffset = new DateTimeOffset(2020, 6, 17, 16, 0, 0, UKoffsetUtc);

// Convert to UTC as a date
var utc = DateTime.SpecifyKind(UKdateTimeOffset.UtcDateTime, DateTimeKind.Utc);

// Get Aus TimeZoneInfo
var AUSTimeZone = TimeZoneInfo.FindSystemTimeZoneById("AUS Eastern Standard Time");

// Check the Aus offset from UTC
var AUSOffset = AUSTimeZone.GetUtcOffset(utc);
Console.WriteLine(AUSOffset); // Output is 10 as expected

// Declare Aus Time as DateTimeOffset
var AUSDateTimeOffset = TimeZoneInfo.ConvertTimeFromUtc(utc, AUSTimeZone);

// The Aus Offset from UTC is not correct 
Console.WriteLine(AUSDateTimeOffset.ToString("dd MM yyyy HH:mm zzz"));

The output is 18 06 2020 01:00 +01:00

Aus are 10 hours ahead of UTC (9 hours ahead of GMT) so the date and time are correct, but not the offset.

How can I get the correct offset in AUSDateTimeOffset?

Answer 1

The error is in this part:

var AUSDateTimeOffset = TimeZoneInfo.ConvertTimeFromUtc(utc, AUSTimeZone);

In that code, utc is a DateTime , and thus the resulting AUSDateTimeOffset is actually a DateTime . Its Kind will be DateTimeKind.Unspecified .

The conversion will have been done correctly, and thus you see the correct date and time in the result. However, the offset is wrong because it is not part of a DateTime . The documentation about the zzz specifier says:

With DateTime values, the "zzz" custom format specifier represents the signed offset of the local operating system's time zone from UTC, measured in hours and minutes. It doesn't reflect the value of an instance's DateTime.Kind property. For this reason, the "zzz" format specifier is not recommended for use with DateTime values.

Thus, the +01:00 is coming from your local time zone, not from the target time zone.

There are a few ways you could fix this:

• You could make AUSDateTimeOffset a DateTimeOffset with the correct offset:

   DateTime AUSDateTime = TimeZoneInfo.ConvertTimeFromUtc(utc, AUSTimeZone); TimeSpan AUSOffset = AUSTimeZone.GetUtcOffset(utc); DateTimeOffset AUSDateTimeOffset = new DateTimeOffset(AUSDateTime, AUSOffset);

• You could use a UTC-based DateTimeOffset instead of a UTC-based DateTime :

   DateTimeOffset utc = UKdateTimeOffset.ToUniversalTime(); DateTimeOffset AUSDateTimeOffset = TimeZoneInfo.ConvertTime(utc, AUSTimeZone);

• You could just convert the orignal DateTimeOffset , as there's no need to convert to UTC first:

   DateTimeOffset AUSDateTimeOffset = TimeZoneInfo.ConvertTime(UKdateTimeOffset, AUSTimeZone);

• As Jimi pointed out in comments, you can even convert without constructing a TimeZoneInfo object at all:

   DateTimeOffset AUSDateTimeOffset = TimeZoneInfo.ConvertTimeBySystemTimeZoneId(UKdateTimeOffset, "AUS Eastern Standard Time");

Any of the above will give the same, correct response.

Answer 2

You can create new offset and use it -

    // Create new offset for UTC
    var AUSOffset = new DateTimeOffset(utc, TimeSpan.Zero);

    // Declare Aus Time as DateTimeOffset
    var AUSDateTimeOffset = UKdateTimeOffset.ToOffset(AUSTimeZone.GetUtcOffset(AUSOffset));                              
    Console.WriteLine(AUSDateTimeOffset.ToString("dd MM yyyy HH:mm zzz"));

Or:

Use ConvertTimeBySystemTimeZoneId as suggested by Jimi in the comment!

    var finalDate = TimeZoneInfo.ConvertTimeBySystemTimeZoneId(UKdateTimeOffset, "AUS Eastern Standard Time");
    Console.WriteLine(finalDate.ToString("dd MM yyyy HH:mm zzz"));

Answer 3

It's easy to get around:

var fromTime = DateTimeOffset.Parse("2020-06-17T16:00:00+01:00");
var timeZoneInfo = TimeZoneInfo.FindSystemTimeZoneById("AUS Eastern Standard Time");//TZConvert.GetTimeZoneInfo("Australia/Sydney");//for Mac
var toTime = fromTime.ToOffset(timeZoneInfo.GetUtcOffset(fromTime.UtcDateTime));
Console.Write(toTime.ToString("yyyy-MM-ddTHH:mm:sszzz")); //2020-06-18T01:00:00+10:00