Are UtcNow and Now the same time? Do they know that they are different?

Question

If I run a fragment, for example:

bool areTheyTheSame = DateTime.UtcNow == DateTime.Now 

what will I get? Does DateTime return its timezone so I can compare?

My particular problem is that I am trying to create a cache-like API. If DateTime AbsoluteExpiration is required, do I need to force my API users to know whether to give me UTC or time in the time zone?

[Edit] This SO question is very important for my problem: Cache.Add absolute expiration - UTC or not?

[Edit] To clarify for future readers, DateTimeKind is what is different. Undefined DateTimeKind is often a problem, and this is what you get, for example, when retrieving from a database. Set DateTimeKind in the DateTime constructor ...

[Edit] JonSkeet wrote a great blog post condemning this behavior and offering a solution: http://noda-time.blogspot.co.uk/2011/08/what-wrong-with-datetime-anyway.html

Answer 1

Did you really try the snippet yourself?

They are different, and direct comparison does not take into account the difference, but you can convert local to UTC by calling ToUniversalTime .

 var now = DateTime.Now; var utcNow = DateTime.UtcNow; Console.WriteLine(now); // 12/07/2010 16:44:16 Console.WriteLine(utcNow); // 12/07/2010 15:44:16 Console.WriteLine(now.ToUniversalTime()); // 12/07/2010 15:44:16 Console.WriteLine(utcNow.ToUniversalTime()); // 12/07/2010 15:44:16 Console.WriteLine(now == utcNow); // False Console.WriteLine(now.ToUniversalTime() == utcNow); // True Console.WriteLine(utcNow.ToUniversalTime() == utcNow); // True 

Answer 2

Also be careful when using the local DateTime and calling .ToUniversalTime() when daylight saving time occurs.

See the note in the comments section here: http://msdn.microsoft.com/en-us/library/system.timezone.touniversaltime.aspx

Answer 3

DateTime.Now returns the system time, and DateTime.UtcNow returns the UTC time.