Clocks in computers have (among others) the following three properties: accuracy, precision, and resolution.

People generally agree on what’s the difference between accuracy and precision/resolution but there seem to be lots of opinions on what’s the difference between precision and resolution and which is which. So I’m going to shamelessly copy a definition I found on Stack Overflow that I’m agreeing with.

• Precision: the amount of information, i.e. the number of significant digits you report. (E.g. I’m 2m, 1.8m, 1.83m, 1.8322m tall. All those measurements are accurate, but increasingly precise.)
• Accuracy: the relation between the reported information and the truth. (E.g. “I’m 1.70m tall” is more precise than “1.8m”, but not actually accurate.)
• Resolution (or Granularity): the smallest time interval that a clock can measure. For example, if you have 1 ms resolution, there’s little point reporting the result with nanosecond precision, since the clock cannot possibly be accurate to that level of precision.

This article will be mainly about resolution (and precision and accuracy to some extend).

DateTime #

C# provides the DateTime type (MSDN) that allows to:

• store a certain point in time
• get the current date and time (via Now or UtcNow)

First, lets take a look at precision: The DateTime type is basically just a 64 bit integer that counts “ticks”. One tick is 100 nanoseconds (or 0.0001 milliseconds) long (MSDN). So DateTime’s precision can be up to 0.0001 milliseconds.

Next, resolution. Basically, we’re asking: “How long does it take for value of DateTime.UtcNow to change?” Lets find out.

The following C# code measures the resolution of DateTime.UtcNow:

Console.WriteLine("Running for 5 seconds...");

var distinctValues = new HashSet<DateTime>();
var sw = Stopwatch.StartNew();

while (sw.Elapsed.TotalSeconds < 5)
{
    distinctValues.Add(DateTime.UtcNow);
}

sw.Stop();

Console.WriteLine("Precision: {0:0.000000} ms ({1} samples)",
                  sw.Elapsed.TotalMilliseconds / distinctValues.Count,
                  distinctValues.Count);

This program records all the different values DateTime.UtcNow returns over the course of 5 seconds. This way, we know how often this value changes per second (or millisecond in this example) and that’s the resolution.

According to MSDN the resolution depends on the operating system but in my tests I found out that the resolution also seems to depend on the hardware (unless newer OS versions have a worse resolution).

Machine	OS	Resolution
Dev Box	Windows 7 x64	1 ms
Laptop	Windows 8 x64	16 ms

High Resolution Clock #

On Windows 8 (or Windows Server 2012) or higher there’s a new API that returns the current time with a much higher resolution:

GetSystemTimePreciseAsFileTime()

Here’s how to use it in C#:

using System;
using System.Runtime.InteropServices;

public static class HighResolutionDateTime
{
    public static bool IsAvailable { get; private set; }

    [DllImport("Kernel32.dll", CallingConvention = CallingConvention.Winapi)]
    private static extern void GetSystemTimePreciseAsFileTime(out long filetime);

    public static DateTime UtcNow
    {
        get
        {
            if (!IsAvailable)
            {
                throw new InvalidOperationException(
                    "High resolution clock isn't available.");
            }

            long filetime;
            GetSystemTimePreciseAsFileTime(out filetime);

            return DateTime.FromFileTimeUtc(filetime);
        }
    }

    static HighResolutionDateTime()
    {
        try
        {
            long filetime;
            GetSystemTimePreciseAsFileTime(out filetime);
            IsAvailable = true;
        }
        catch (EntryPointNotFoundException)
        {
            // Not running Windows 8 or higher.
            IsAvailable = false;
        }
    }
}

Using the same test code as above but using HighResolutionDateTime.UtcNow as input (instead of DateTime.UtcNow) leads to:

Machine	OS	Resolution
Dev Box	Windows 7 x64	n/a
Laptop	Windows 8 x64	0.0004 ms

So, on my laptop the resolution increased by a factor of 40000.

Accuracy #

To complete this article, lets also talk about accuracy.

DateTime.UtcNow and HighResolutionDateTime.UtcNow are both very accurate. The first one has lower resolution, the second one has higher resolution.

There’s also Stopwatch in C#. Stopwatch has a high resolution. Using Stopwatch.ElapsedTicks as input for resolution measure code from above, I got these results:

Machine	OS	Resolution
Dev Box	Windows 7 x64	0.0004 ms
Laptop	Windows 8 x64	0.0004 ms

However, Stopwatch is not very accurate. On my laptop it drifts by 0.2 ms per second, i.e. it gets less accurate over time.

Here’s how to measure the drift/accuracy loss:

var start = HighResolutionDateTime.UtcNow;
var sw = Stopwatch.StartNew();

while (sw.Elapsed.TotalSeconds < 10)
{
    DateTime nowBasedOnStopwatch = start + sw.Elapsed;
    TimeSpan diff = HighResolutionDateTime.UtcNow - nowBasedOnStopwatch;

    Console.WriteLine("Diff: {0:0.000} ms", diff.TotalMilliseconds);

    Thread.Sleep(1000);
}

This gives me an output like this:

Diff: 0,075 ms
Diff: 0,414 ms
Diff: 0,754 ms
Diff: 0,924 ms
Diff: 1,084 ms
Diff: 1,247 ms
Diff: 1,409 ms
Diff: 1,571 ms
Diff: 1,734 ms
Diff: 1,898 ms

As you can see, the difference increases over time. Thus, Stopwatch becomes less accurate over time.