Now versus UtcNow

This is a minor topic, but it was such a drastic difference when I found out about it, that it’s definitely worth sharing. Consider the following code:

 var begin = System.DateTime.UtcNow;
 for (var i = 0; i < 10000000; i++)
 {
 var foo = System.DateTime.Now;
 }
 System.Console.WriteLine(System.DateTime.UtcNow.Subtract(begin).TotalMilliseconds);
 System.Console.ReadLine();

The result on my machine was 9150 milliseconds. Now try this:

 var begin = System.DateTime.UtcNow;
 for (var i = 0; i < 10000000; i++)
 {
 var foo = System.DateTime.UtcNow;
 }
 System.Console.WriteLine(System.DateTime.UtcNow.Subtract(begin).TotalMilliseconds);
 System.Console.ReadLine();

The result was 152 milliseconds. Really. Not kidding. Yes, there are better ways of measuring, but that one worked well enough in this case. Apparently, the framework uses UTC natively, and then converts to local times when it matters. And that conversion comes with a huge cost!

Now you know. And knowing is half the battle.

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