C# DateTime.Now precision

Why would DateTime.Now be made less precise than what most CPU clocks could handle?

A good clock should be both precise and accurate; those are different. As the old joke goes, a stopped clock is exactly accurate twice a day, a clock a minute slow is never accurate at any time. But the clock a minute slow is always precise to the nearest minute, whereas a stopped clock has no useful precision at all.

Why should the DateTime be precise to, say a microsecond when it cannot possibly be accurate to the microsecond? Most people do not have any source for official time signals that are accurate to the microsecond. Therefore giving six digits after the decimal place of precision, the last five of which are garbage would be lying.

Remember, the purpose of DateTime is to represent a date and time. High-precision timings is not at all the purpose of DateTime; as you note, that's the purpose of StopWatch. The purpose of DateTime is to represent a date and time for purposes like displaying the current time to the user, computing the number of days until next Tuesday, and so on.

In short, "what time is it?" and "how long did that take?" are completely different questions; don't use a tool designed to answer one question to answer the other.

Thanks for the question; this will make a good blog article! :-)

DateTime's precision is somewhat specific to the system it's being run on. The precision is related to the speed of a context switch, which tends to be around 15 or 16 ms. (On my system, it is actually about 14 ms from my testing, but I've seen some laptops where it's closer to 35-40 ms accuracy.)

Peter Bromberg wrote an article on high precision code timing in C#, which discusses this.

I would like a precise Datetime.Now :), so I cooked this up:

public class PreciseDatetime
{
    // using DateTime.Now resulted in many many log events with the same timestamp.
    // use static variables in case there are many instances of this class in use in the same program
    // (that way they will all be in sync)
    private static readonly Stopwatch myStopwatch = new Stopwatch();
    private static System.DateTime myStopwatchStartTime;

    static PreciseDatetime()
    {
        Reset();

        try
        {
            // In case the system clock gets updated
            SystemEvents.TimeChanged += SystemEvents_TimeChanged;
        }
        catch (Exception)
        {                
        }
    }

    static void SystemEvents_TimeChanged(object sender, EventArgs e)
    {
        Reset();
    }

    // SystemEvents.TimeChanged can be slow to fire (3 secs), so allow forcing of reset
    static public void Reset()
    {
        myStopwatchStartTime = System.DateTime.Now;
        myStopwatch.Restart();
    }

    public System.DateTime Now { get { return myStopwatchStartTime.Add(myStopwatch.Elapsed); } }
}