System.DateTime.Now is giving bad results

Question

The main logic of a utility tool is in a function like this:

private void Run()
{
    DateTime startTime = DateTime.Now;
    Prepare();
    Search();
    Process();
    DateTime endTime = DateTime.Now;
    TimeSpan duration = endTime.Subtract(startTime);
    Console.WriteLine("Run took {0:00}:{1:00}:{2:00}",
        (int)duration.TotalHours, duration.Minutes, duration.Seconds);
}

When I run this I can see with my own eyes it is taking at least 5 seconds (Process() method spews console output which I can observe happening for 5-6s). But it reports "Run took 00:00:01".

I don't expect time to have microsecond precision but why is it so totally inaccurate here?

Update: Following advice I also ran a StopWatch over the same period and compared against subtracting two DateTime, and also debugged the code. The two methods agree to a fraction of a second... StopWatch in the debugger had 1139 milliseconds. My hypothesis is that somehow time writing to the console is not included but I have no way to back that up (or disprove it).

i guess you should use a [Stopwatch](http://msdn.microsoft.com/en-us/library/system.diagnostics.stopwatch%28v=vs.80%29.aspx) and also have a look [here](http://stackoverflow.com/questions/8754018/recommended-method-to-calculate-the-difference-between-two-timespans/8754105#8754105) — V4Vendetta, Mar 13 '13 at 11:45
is the value in the duration TimeSpan around 5-6 sec, and its an issue of incorrect displaying??? — Fredrik, Mar 13 '13 at 11:47
i run you code and i didnt find any problem in this,. try to debug code — Sigar Dave, Mar 13 '13 at 11:54
See also - http://codeofmatt.com/2013/04/25/the-case-against-datetime-now/ — Matt Johnson-Pint, May 10 '13 at 01:53

score 2 · Answer 1 · answered Mar 13 '13 at 11:49

2

To become confident in what you see is what really happens, write for test sake:

DateTime startTime = DateTime.Now;
Thread.Sleep(5000);
DateTime endTime = DateTime.Now;
TimeSpan duration = endTime.Subtract(startTime);
Console.WriteLine("Run took {0:00}:{1:00}:{2:00}",
        (int)duration.TotalHours, duration.Minutes, duration.Seconds);

Also, it's better to use Stopwatch class for your purposes

answered Mar 13 '13 at 11:49

Dima

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On longer runs it seems less inaccurate. I was even wondering if the console is lagging a few seconds behind the actual ed of the program... it writes a lot of lines to the console. I'll maybe compare `StopWatch` and see if the same results are seen. – Mr. Boy Mar 13 '13 at 12:01

Jim Mischel · Accepted Answer · 2013-03-13T12:36:19.373

2

I doubt that the problem is with DateTime. Likely, the program finished and sent the output to the console buffer, which is taking its sweet time actually displaying it. What you're seeing is output lag.

edited Mar 13 '13 at 12:36

answered Mar 13 '13 at 12:29

Jim Mischel

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score 1 · Answer 3 · answered Mar 13 '13 at 11:51

1

Why not use Stopwatch?

Stopwatch ss = new Stopwatch();
ss.Start();
// Some quantity of work..... 
ss.Stop();

Console.WriteLine("Elapsed time: {0}", ss.Elapsed.TotalMilliseconds);

answered Mar 13 '13 at 11:51

Travis G

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score 0 · Answer 4 · answered Mar 13 '13 at 11:46

0

DateTime isn't very accurate for this kind of measurement. You should use the stopwatch class.

http://msdn.microsoft.com/en-us/library/system.diagnostics.stopwatch.aspx

answered Mar 13 '13 at 11:46

stevepkr84

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Not accurate to the nearest _second_? I find that very hard to believe. – Mr. Boy Mar 13 '13 at 11:56
@John, I do agree. Perhaps you can try it turning off the console output in the main methods. – stevepkr84 Mar 13 '13 at 12:15

score 0 · Answer 5 · answered Mar 13 '13 at 12:06

0

DateTime startTime = DateTime.Now;
Thread.Sleep(5000);
DateTime endTime = DateTime.Now;
TimeSpan duration = endTime.Subtract(startTime);
Console.WriteLine(duration.Seconds);

Prints "5". Are you sure your test case is what it is?

answered Mar 13 '13 at 12:06

CodeCaster

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Your test case isn't the same as mine. Write a few thousand lines to the console and time it by hand Vs what the timer tells you and let me know what you see? – Mr. Boy Mar 13 '13 at 14:57
@John yes, the console lags. – CodeCaster Mar 13 '13 at 18:02
Thanks. That seems fairly conclusive then. – Mr. Boy Mar 13 '13 at 18:29

System.DateTime.Now is giving bad results

5 Answers5