Counting program running time

Sometims we want to know how fast our program running,
I found two methods on stack overflow.

DateTime dt = DateTime.Now; 
for (.......) { }
TimeSpan ts = DateTime.Now - dt;
Console.WriteLine("total runtime: " + ts.ElapsedMilliseconds.ToString() + "ms");
or (according to MSDN) if you need better resolution
Stopwatch stopWatch = new Stopwatch(); 
stopWatch.Start(); 
for (.......) { } 
stopWatch.Stop(); 
Console.WriteLine("total runtime: " + stopWatch.ElapsedMilliseconds.ToString() + "ms");