Timer surprises, and how to avoid them

//*******************************************************************
// LP TimerTest
//
// a test application for .NET timers
//
// Copyright: Luc Pattyn, January 2007
//
// This code is freely available for any non-commercial use.
//
//*******************************************************************
using System;
using System.Text;
using System.Threading;

namespace TimerTest {
	// This class contains the Threading Timer experiment
	public class ThreadTimerTest {
		private Stringer logger;
		private System.Threading.Timer threadTimer;
		private int count;
		private int maxCount;
		private int interval;
		private DateTime start;
		private DateTime stop;

		public ThreadTimerTest(Stringer logger) {
			this.logger=logger;
		}

		private void log(string s) {
			if(logger!=null) logger(s);
		}

		public void Run(int maxCount, int interval) {
			count=0;
			this.maxCount=maxCount;
			this.interval=interval;
			log("Observing "+maxCount+" consecutive "+interval+" msec intervals for a System.Threading.Timer");
			threadTimer=new System.Threading.Timer(new TimerCallback(timerCallback),
				null, interval, interval);
			start=DateTime.Now;
			log("    start = "+start.ToString("HH:mm:ss.fff"));
		}

		private void timerCallback(object argNotUsed) {
			// warning: runs on another thread, so form.log and form.Done must be Invoked
			count++;
			if(count>=maxCount) {
				threadTimer.Dispose();
				stop=DateTime.Now;
				log("    stop  = "+stop.ToString("HH:mm:ss.fff"));
				log("    Thread Timer has finished");
				TimeSpan span=stop.Subtract(start);
				double msec=span.TotalMilliseconds;
				log("    got "+count+" ticks in "+msec+" msec");
				msec/=count;
				log("    average interval was "+msec+" msec");
				log("    but timer interval was "+interval+" msec");
				log("conclusions:");
				log(" - effective timer interval is not less than requested value");
				log(" - effective timer interval may be greater than requested value");
				log(" - effective timer interval corresponds to first multiple of");
				log("   system timer's period that is >= requested value");
				log("~");
			}
		}
	}
}