Precision and accuracy of DateTime

Precision and accuracy of DateTime

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StopwatchThe DateTime struct represents dates as a 64 bit number that measures the number of “ticks” since a particular start date. Ten million ticks equals one second.

That’s a quite high degree of precision. You can represent dates and times to sub-microsecond accuracy with a DateTime, which is typically more precision than you need. Not always, of course; on modern hardware you can probably execute a couple hundred instructions in one tick, and therefore if you want timings that are at the level of precision needed to talk about individual instructions, the tick is too coarse a measure.

The problem that arises with having that much precision is of course that it is very easy to assume that a given value is as accurate as it is precise. But that’s not warranted at all! I can represent my height in a double-precision floating point number as 1.799992352094 metres; though precise to a trillionth of a metre, it’s only accurate to about a hundredth of a metre because I do not have a device which can actually measure my height to a trillionth of a meter, or even a thousandth of a metre. There is way more precision than accuracy here.

The same goes for dates and times. Your DateTime might have precision down to the sub-microsecond level, but does it have accuracy? I synchronize my computers with fairly regulary. But if I don’t do so, their clocks wander by a couple of seconds a year typically. Suppose my clock loses one second a year. There are 31.5 million seconds in a year and 10 million ticks in a second, so therefore it is losing one tick every 3.15 seconds. Even if my clock was miraculously accurate down to the level of a tick at some point, within ten seconds, it’s already well off. Within a day much of the precision will be garbage.

If you do a little experiment you’ll see that the operating system actually gives you thousands of times less accuracy than precision when asked “what time is it?”

long ticks = DateTime.Now.Ticks;
    if (ticks != DateTime.Now.Ticks)
        ticks = DateTime.Now.Ticks;

On my machine this says “same” eight or nine times, and then suddenly the Ticks property jumps by about 160000, which is 16 milliseconds, a 64th of a second. (Different flavours of Windows might give you different results, depending on details of their thread timing algorithms and other implementation details.)

As you can see, the clock appears to be precise to the sub-microsecond level but it is in practice only precise to 16 milliseconds. (And of course whether it is accurate to that level depends on how accurately the clock is synchronized to the official time signal.)

Is this a flaw in DateTime.Now? Not really. The purpose of the “wall clock” timer is to produce dates and times for typical real-world uses, like “what time does Doctor Who start?” or “when do we change to daylight savings time?” or “show me the documents I edited last Thursday after lunch.”  These are not operations that require submicrosecond accuracy.

(And incidentally, in VBScript the “wall clock” timer methods built in to the language actually round off times we get from the operating system to the nearest second, not the nearest 64th of a second.)

In short, the question “what time is it?” really should only be answered to a level of precision that reflects the level of accuracy inherent in the system. Most computer clocks are not accurately synchronized to even within a millisecond of official time, and therefore precision beyond that level of accuracy is a lie. It is rather unfortunate, in my opinion, that the DateTime structure does surface as much precision as it does, because it makes it seem like operations on that structure ought to be accurate to that level too. But they almost certainly are not that accurate.

Now, the question “how much time has elapsed from start to finish?” is a completely different question than “what time is it right now?” If the question you want to ask is about how long some operation took, and you want a high-precision, high-accuracy answer, then use the StopWatch class. It really does have nanosecond precision and accuracy that is close to its precision.

Remember, you don’t need to know what time it is to know how much time has elapsed. Those can be two different things entirely.

  • "I don't know that I would go so far as to say DateTime supporting more preciscion than is provided by the hosting hardware\platform is a bad thing.

    At no extra cost to developers DateTime can support more precise hosts in the future, in my book that's a good thing."

    Yeah, big ++ to this (and glad that Datetime2 exists now in sql server)

    It is entirely possible to get better precision from other devices, having the precision in the standard struct is so much nicer, and meant that our use of such devices didn't involve a tedious replacement of DateTime with some other one everywhere... I viewed MS's provision of DateTime with this level of precision in storage as very forward thinking and extremely sensible, I'm surprised you don't think it was a good idea.

    Note that if MS ever start using the HPET timer in modern systems for the system time (as many linux distributions now do) they will almost instantly start hitting well beyond millisecond level precision. Even when the accuracy is off the offset is likely to remain within this level of precision over the course of a day so it is still useful. In fact given there truly being no one true time in a relativistic sense the ability to have decent local precision right now is really why having it baked into the struct is a good thing.

    The next issue will be people assuming this level of precision in DateTimes/TimeSpans means that they can request waits/pauses of OS level constructs with that level of granularity. That they can do this already with milliseconds being the precision 'exposed' by Sleep() but the scheduler only giving 10-15ms of actually granularity as it stands suggests that it is these constructs that need either better documentation or more 'obviousness' in the type of their arguments.

  • Chris:

    PBS (Public Broadcasting Station) has affiliates all over the US.  I have fond memories of donation drives where the bonus for making a $200 or more donation was a rather long scarf...  I almost stole my father's checkbook.  

    It took off in the US in geek cricle during the Tom Baker era, mostly.  Among the geeks it was a huge phenomenon.  They attempted to make a new, American Dr. in the late 90s in the US, it bombed thanks to it being far too different from the British version.  It had a pilot and got buried.  

    Syfy Channel (formerly Sci Fi Channel) ran the latest series of the Dr. about a season behind.  BBC US will show it soon.  I am guessing SyFy will snatch it up and show it behind the times a bit too.  

    (I loved Torchwood as well, and saw it on BBC US)

    As for the actual comment, I know when Dr. Who is on.  Whenever I want it to be thanks to Tivo!  :)

  • > what time does Doctor Who start?

    Given The Doctor can't distinguish 5 minutes and 10 years, I think there is a whole different accuracy argument when it comes to Time Lords.

    (And the answer is 2010-04-10T18:15 BST.)

  • ShuggyCoUk: I recall hearing that they intentionally don't have a higher-resolution clock because it uses too much power. When a CPU has to wake up 1000 times per second to increment the clock ticks, it can't spend much time in low-power sleep states. Keeping the clock at only 64 Hz enables lower power consumption and longer battery life. In fact, in Win 7 they went to great lengths to reduce timer interrupts (implementing timer coalescing).

    pete.d: You seem to have a different definition of "guarantee" than I do.

    Chris: I grew up watching Dr. Who in the early 80s in America.

  • Frustratingly, BBC America has decided to air new Dr Who episodes approximately two weeks later in the US than in the UK.

    Thereby guaranteeing that half their audience will torrent it and they'll lose a ton of ad revenue.

    For those of us who wait to watch it until it actually airs, that's doubly frustrating, because most of our friends have already seen it...

  • I think most of these issues would go away when they finally start installing atomic clocks on the motherboard.

  • "On my machine this says “same” eight or nine times, and then suddenly the Ticks property jumps by about 160000, which is 16 milliseconds, a 64th of a second"

    Weired. On my PC (old core 2 duo) it jums by same ~160000, but it prints ~3600 of "same" in between when I run it from command line, and ~500 when I run it from Visual Studio 2005, which is 7x slow down, which is descent. Eric is talking about 60x slow-down comparing to Visual Studio 2005, which is scary.

    Eric, do you use the latest Visual Studio? Is there something fundamentally wrong with output window performance? Or am I missing something?

  • Unfortunately, Stopwatch does not exist in Silverlight 3 or 4, and the only way I see to implement it is using DateTime.Now. Is there another way to get a more precise elapsed time measurement on Silverlight?

    You'll have to ask someone who is an expert on Silverlight. I wouldn't know. -- Eric

  • "It is rather unfortunate, in my opinion, that the DateTime structure does surface as much precision as it does, because it makes it seem like operations on that structure ought to be accurate to that level too."

    But, Eric, it's still nice to have a convenient type for holding and manipulating high precision time values. The time values themselves may come from an external source.

  • @M. McCulloch:

    "But, Eric, it's still nice to have a convenient type for holding and manipulating high precision time values. The time values themselves may come from an external source."

    You take the words right out of my mouth, the DateTime struct is in no way bound to DateTime.Now (system time), DateTime.Now just happens to be one of the users of the struct.

  • @francis d

    You're not considering the time it takes to "DO it"; thus you're accumulating drift each time you perform a "DO it" operation.

  • Hi Eric!

    What you write is right, but I'd see the accuracy of DateTime fron another viewpoint. I've build an app displaying historical data (generated by a SCADA), that records thousands of samples marked by a timestamp. Well, obviously neither our hw is capable to guarantee tenths of us, but I may have to record samples generated by a 10MHz (and over) source, for example. This isn't so special case...In that way, probably neither the 0.1 us resolution would be enough.

    Good article, anyway.


  • I actually receive time values with this level of precision from my instrumentation and it is nice that I don't need any special constructs to deal with them in .NET after parsing them into the appropirate DateTime or TimeSpan objects.

    So I can use the standard DateTime.Add/Subtract/etc to group sets of 1ms pulses and then if I do a properly formatted ToString I'll get the 'wall-clock-time' for the first line in the report.

    I'll tell ya, it's not quite as fun trying to do those kinds of things in C++ or other frameworks

  • francis: its because your friends process doesn't start waiting 1 minute from the beginning of the minute, but AFTER the process runs, which is indeterminate, ie, the process that runs may take 1 second one time (no work performed) then 30 seconds the next time.  Your 'drift' is due to the process run time, and using a method like "Sleep(1min)" will cause your process to drift indeterminately because of it.

    Process A

    12:00:00 run.

    12:01:01 first run complete [WAIT 1 MINUTE]

    12:02:01 run.

    12:02:31 second run complete [WAIT 1 MINUTE]

    12:03:31 run.

  • Do not use DateTime; use DateTimeOffset instead.

    "DateTimeOffset should be considered the default date and time type for application development"

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