Are UtcNow and Now the same datetime? Do they know they're different?

Question

If I run a snippet like:

bool areTheyTheSame = DateTime.UtcNow == DateTime.Now

what will I get? Does the DateTime returned know its timezone such that I can compare?

My specific problem is that I'm trying to build a cache-like API. If it takes a DateTime AbsoluteExpiration, do I have to enforce that users of my API know whether to give me a UTC time or a timezone-based time?

[Edit] This SO question is extremely relevant to my issue as well: Cache.Add absolute expiration - UTC based or not?

[Edit] Just to clarify for future readers, the DateTimeKind is what is different. The Undefined DateTimeKind's are often a problem, which is what you get when you pull one out of a database, for instance. Set the DateTimeKind in the DateTime constructor...

[Edit] JonSkeet wrote a lovely 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 actually try the snippet yourself?

They're different, and a straight comparison doesn't account for 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