How to compare DateTime values, taking into account timezone?

Question

I have two DateTime variables. Each has a timezone stored in the variable so that when I ToString with format including zzz I get a string including +01:00.

At design time I do not know what the timezones will be and I am expecting the variables to have different timezones from each other.

I want to compare the two DateTime values so that I know which is more recent.

For example, if variable A is 2015-07-04T02:00:00+03:00 and variable B is 2015-07-03T18:00:00-07:00 then B > A.

What do I write in C# to tell me this? (I would prefer not to use a third party library.)

(To the SO question-closing zealots: I have spent several hours investigating this using Google, MSDN and SO and am confused. I cannot find a very similar question to this on SO. I am confident that answers to this question will help others.)

Answer 1

You said:

I have two DateTime variables. Each has a timezone stored in the variable so that when I ToString with format including zzz I get a string including +01:00.

This is a common misunderstanding. DateTime doesn't have a time zone stored in the variable. It only has a Kind property, which is of type DateTimeKind, and can be either Utc, Local, or Unspecified.

When calling ToString, the zzz format specifier uses the Kind property to determine which offset to display.

• When the Kind is DateTimeKind.Utc, the offset is always +00:00.

• When the Kind is DateTimeKind.Local, the offset is determined from the local time zone on the computer where the code is executing. For example, my computer is set to US Pacific time, so the offset will be either -08:00 or -07:00 depending on whether daylight saving time is in effect or not.

• When the Kind is DateTimeKind.Unspecified, the behavior is the same as if it were Local. Keep in mind that other methods treat Unspecified in different ways - this is just the particular behavior of the zzz specifier.

MSDN actually says:

For this reason, the "zzz" format specifier is not recommended for use with DateTime values.

Going back to your question:

At design time I do not know what the timezones will be and I am expecting the variables to have different timezones from each other.

Then you cannot use DateTime. You should instead use DateTimeOffset, as it retains a specific time zone offset instead of using a DateTimeKind.

For example, if variable A is 2015-07-04T02:00:00+03:00 and variable B is 2015-07-03T18:00:00-07:00 then B > A. What do I write in C# to tell me this?

DateTimeOffset a = DateTimeOffset.Parse("2015-07-04T02:00:00+03:00");
DateTimeOffset b = DateTimeOffset.Parse("2015-07-03T18:00:00-07:00");

bool result = b > a;  // true

See also: DateTime vs DatetimeOffset

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Furthermore

As Gustav pointed out, you can use just DateTime, as long as you convert back to universal time before comparing. This works due to DateTime's hidden fourth state (more here). The state is set properly during parsing, and is taken into account when ToUniversalTime is called. Then comparison has valid UTC times to operate from.

DateTime A = DateTime.Parse("2015-11-01T01:00:00-07:00");
DateTime B = DateTime.Parse("2015-11-01T01:00:00-08:00");

Console.WriteLine(A.ToUniversalTime().ToString("'A: 'yyyy'-'MM'-'dd hh:mm:ss"));
Console.WriteLine(B.ToUniversalTime().ToString("'B: 'yyyy'-'MM'-'dd hh:mm:ss"));
Console.WriteLine( B.ToUniversalTime() > A.ToUniversalTime() );
Console.WriteLine( B > A );

And the result:

A: 2015-11-01 08:00:00
B: 2015-11-01 09:00:00
True
False

If your local time zone is set to Pacific Time, you'll get the above results. However, if it's set to something else - it's possible you will get True for the last result, because the values may have been parsed to different local times in your time zone, even though they'd be the same local time in the Pacific time zone.

Using DateTimeOffset is still simpler, going through less conversions, and not being affected by the local time zone.