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    <p>Clock drift. Very typical for trivial timers. The reason for this is that they are typically implemented using a sleep function. A sleep function is always guaranteed to sleep for at least the time specified but not guaranteed to not sleep for more time than that, in practice it's always accumulating a drift.</p> <p>There are ways to write timer that compensate for the drift and on average hit the target.</p> <p>One of my favorite timers are fixed step ones, that have a discreet tick. It's very simple and looks like this:</p> <pre><code>var t = DateTime.Now + TimeSpan.FromSeconds(1); for (;;) { if (DateTime.Now &gt;= t) { t += TimeSpan.FromSeconds(1); // Tick! } } </code></pre> <p>That's a crude but effective timer, this following one is an example of a clock I built for a WPF timer where the built in timer was suffering from drifting. This timer is a lot more complex and it does not hog your CPU. But it clearily illustrates typical issues that timers have.</p> <p>The OnTimerTick here is using a built in timer that suffers from drift but it's adjusting the interval to compensate for the drift.</p> <pre><code>/// &lt;summary&gt; /// Occurs when the timer interval has elapsed. /// &lt;/summary&gt; public event EventHandler Tick; DispatcherTimer timer; public bool IsRunning { get { return timer.IsEnabled; } } long step, nextTick, n; public TimeSpan Elapsed { get { return new TimeSpan(n * step); } } public FixedStepDispatcherTimer(TimeSpan interval) { if (interval &lt; TimeSpan.Zero) { throw new ArgumentOutOfRangeException("interval"); } this.timer = new DispatcherTimer(); this.timer.Tick += new EventHandler(OnTimerTick); this.step = interval.Ticks; } TimeSpan GetTimerInterval() { var interval = nextTick - DateTime.Now.Ticks; if (interval &gt; 0) { return new TimeSpan(interval); } return TimeSpan.Zero; // yield } void OnTimerTick(object sender, EventArgs e) { if (DateTime.Now.Ticks &gt;= nextTick) { n++; if (Tick != null) { Tick(this, EventArgs.Empty); } nextTick += step; } var interval = GetTimerInterval(); Trace.WriteLine(interval); timer.Interval = interval; } public void Reset() { n = 0; nextTick = DateTime.Now.Ticks; } public void Start() { var now = DateTime.Now.Ticks; nextTick = now + (step - (nextTick % step)); timer.Interval = GetTimerInterval(); timer.Start(); } public void Stop() { timer.Stop(); nextTick = DateTime.Now.Ticks % step; } </code></pre>
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    1. POSystem.Timer elapsed event seems to fire late for short intervals in .Net
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    1. POSystem.Timer elapsed event seems to fire late for short intervals in .Net
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    1. COInteresting, I come from a background of embedded systems. In those you have finite clock resolution, but the accuracy is as good as the clock frequency. I guess I was assuming these timers were purely counting the processor clock. To a certain extent I guess they are but then you have the .NET overhead to get back into your execution.
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    2. COThis stuff holds for native development as well and should not be attributed to any managed code overhead. Anything similar to a loop with a `sleep(1000);` at the end will suffer from the same problem due how the operating system suspend/resume the execution of a thread. This problem has to do with thread handling and remains true for any preemptive multithreaded system where the operating system is switching between concurrent workloads.
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