是UtcNow现在同样的日期时间？他们知道他们有什么不同？ [英] Are UtcNow and Now the same datetime? Do they know they're different?

问题描述

如果我运行像一个片段：

 布尔areTheyTheSame = DateTime.UtcNow == DateTime.Now 
  

我将要得到什么？请问日期时间返回知道它的时区，这样我可以比较？

我的具体问题是，我想建立一个缓存一样API。如果需要一个DateTime AbsoluteExpiration，我必须执行我的API的用户知道能否给我一个UTC时间或基于时区时间？

这太问题是我的问题高度相关，以及： Cache.Add绝对过期 - 基于UTC还是不

只是为了澄清对未来的读者，在DateTimeKind是有什么不同？未定义DateTimeKind的往往是一个问题，这是当你拉一明一数据库，比如你会得到什么。设置在DateTimeKind DateTime构造...

JonSkeet写了一个可爱的博客文章谴责这种行为，并提供一个解决方案：的 http://noda-time.blogspot.co.uk/2011/08 /what-wrong-with-datetime-anyway.html

解决方案

你真的自己尝试片段<？ / p>

他们是不同的，直的比较不占的差异，但你可以通过调用的 ToUniversalTime 。

  VAR现在= DateTime.Now; 
 VAR utcNow = DateTime.UtcNow; 
 
 Console.WriteLine（现在的）; // 12/07/2010 16时44分16秒
 Console.WriteLine（utcNow）; // 12/07/2010 15时44分16秒
 Console.WriteLine（now.ToUniversalTime（））; // 12/07/2010 15时44分16秒
 Console.WriteLine（utcNow.ToUniversalTime（））; // 12/07/2010 15时44分16秒
 
 Console.WriteLine（现== utcNow）; //假
 Console.WriteLine（now.ToUniversalTime（）== utcNow）; //真
 Console.WriteLine（utcNow.ToUniversalTime（）== utcNow）; //真
  

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

解决方案

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

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