Execution Timing, DateTime Ticks Resolution
The Ticks property of a TimeSpan produced by subtracting two DateTime.Now value has a resolution of 156247.
There are 10,000,000 ticks per second (TimeSpan.TicksPerSecond), so the resolution is 15.6247 milliseconds.
The Environment.TickCount property counts millisecond ticks and has a resolution of between 15 and 16 milliseconds.
Modified an example of using QueryPerformanceCounter Win32 API to create a high resolution timer for use in C#.