Time elapse computation in milliseconds C#

using System.Diagnostics;

//...

var stopwatch = new Stopwatch();
stopwatch.Start();
for (int i = 0; i < N_ITER; i++) {
    // cpu intensive sequence
}
stopwatch.Stop();
elapsed_time = stopwatch.ElapsedMilliseconds;

Answer EDITED based on comments

This answer is only trying to count the total elapsed Milliseconds between two times, where the times are derived directly from DateTime.Now. As per the conversation, it's understood that DateTime.Now is vulnerable to outside influences. Hence the best solution would be to use the Stopwatch class. Here's a link that better explains (IMO) and discusses the performance between DateTimeNow, DateTime.Ticks, StopWatch.

Original Answer

The way you cast it into a int is the issue. You need better casting and extra elements :) This may looks simple compared to an efficient timer. But it works:

DateTime startTime, endTime;
startTime = DateTime.Now;

//do your work

endTime = DateTime.Now;
Double elapsedMillisecs = ((TimeSpan)(endTime - startTime)).TotalMilliseconds;

There is a reference on the web, you may want to check out as well.

You're looking for the Stopwatch class. It is specifically designed to bring back high-accuracy time measurements.

var stopwatch = new Stopwatch();

stopwatch.Start();
for (int i = 0; i < N_ITER; i++)
{
     // cpu intensive sequence
}
stopwatch.Stop();

var elapsed = stopwatch.ElapsedMilliseconds;