Ignore milliseconds when comparing two datetimes

Question

I am comparing the LastWriteTime of two files, however it is always failing because the file I downloaded off the net always has milliseconds set at 0, and my original file has an actual value. Is there a simple way to ignore the milliseconds when comparing?

Here's my function:

//compare file's dates
public bool CompareByModifiedDate(string strOrigFile, string strDownloadedFile)
{
     DateTime dtOrig = File.GetLastWriteTime(strOrigFile);
     DateTime dtNew = File.GetLastWriteTime(strDownloadedFile);
            
     if (dtOrig == dtNew)
        return true;
     else
        return false;
}

Duplicate: [How to truncate milliseconds off of a .NET DateTime](http://stackoverflow.com/questions/1004698/) — Jeppe Stig Nielsen, Jul 28 '16 at 11:39

score 124 · Answer 1 · edited Jun 07 '18 at 05:19

124

I recommend you use an extension method:

public static DateTime TrimMilliseconds(this DateTime dt)
{
    return new DateTime(dt.Year, dt.Month, dt.Day, dt.Hour, dt.Minute, dt.Second, 0, dt.Kind);
}

then its just:

if (dtOrig.TrimMilliseconds() == dtNew.TrimMilliseconds())

edited Jun 07 '18 at 05:19

Eternal21

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answered Aug 11 '11 at 15:56

Dean Chalk

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Perhaps preserve the `DateTimeKind` as well by adding `dt.Kind` to end of ctor – Luke Hutton Oct 23 '14 at 18:27
4

return new DateTime(dt.Year, dt.Month, dt.Day, dt.Hour, dt.Minute, dt.Second, 0, dt.Kind); – Adamy Jun 12 '15 at 02:46

Peter Ivan · Answer 2 · 2016-07-18T07:08:57.397

56

Care should be taken, if dt has non-zero microseconds (fractions of millis). Setting only milliseconds to zero is not enough.
To set millis and below to zero (and get a succesfull comparison), the code would be:

dt = dt.AddTicks(-dt.Ticks % TimeSpan.TicksPerSecond); // TimeSpan.TicksPerSecond=10000000

edited Jul 18 '16 at 07:08

answered Jul 19 '12 at 09:43

Peter Ivan

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This is the best solution. Fast, and preserves `dt.Kind`. I know it gives the same in all cases, but to me it is more natural to use `dt = dt.AddTicks(-(dt.Ticks % TimeSpan.TicksPerSecond));`, i.e. use `%` operator first, and then negate. That is because the behavior of `%` with a negative first operand seems a bit confusing to me, so I prefer my version where `%` operates on two positive operands. – Jeppe Stig Nielsen Jul 28 '16 at 11:23
@JeppeStigNielsen: Your code is exactly the same as mine. According to operator precedence, you multiply and/or divide and just then add/negate. So the parentheses change nothing, just clarify the well-established logic. – Peter Ivan Jul 29 '16 at 11:22
No that is incorrect. In rare cases (that cannot occur in your use above, so your use is fine), there is a difference. Consider this code (where `a` and `b` must not be declared `const`): `long a = long.MinValue; long b = 10L; long x = (-a) % b; long y = -(a % b); long z = -a % b;` Here the value of `x` will be negative, because negation of `a` gives `a` again, and a negative value `%`-ed on a positive value gives a negative value. But the value of `y` is positive, because this time we negate a negative result between `-10` and `0`, and that gives a positive number. Now check `z` to see it! – Jeppe Stig Nielsen Jul 29 '16 at 12:40

score 47 · Accepted Answer · answered Aug 11 '11 at 15:53

47

Create a new DateTime value with the milliseconds component set to 0:

dt = dt.AddMilliseconds(-dt.Millisecond);

answered Aug 11 '11 at 15:53

dtb

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Warning: this won't work when the `DateTime` has non-zero microseconds. See @PeterIvan's or @DeanChalk's answers. – TheCloudlessSky Sep 13 '13 at 11:47

score 32 · Answer 4 · answered Aug 11 '11 at 16:01

32

TimeSpan difference = dtNew - dtOrig;
if (difference >= TimeSpan.FromSeconds(1))
{
    ...
}

answered Aug 11 '11 at 16:01

Paul Ruane

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This is the way to go IMHO. – Larry Feb 09 '16 at 10:13

Sanjay Manohar · Answer 5 · 2011-08-11T16:13:39.550

18

You can subtract them, to get a TimeSpan.

Then use TimeSpan.totalSeconds()

edited Aug 11 '11 at 16:13

answered Aug 11 '11 at 15:55

Sanjay Manohar

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score 8 · Answer 6 · answered Aug 11 '11 at 16:28

This is overkill for a single Truncate, but if you have several and of various types you could do this using the generalized Extension Method below:

DateTime dtSecs = DateTime.Now.TruncateTo(Extensions.DateTruncate.Second);
DateTime dtHrs  = DateTime.Now.TruncateTo(Extensions.DateTruncate.Hour);

More general Use Extension method:

    public static DateTime TruncateTo(this DateTime dt, DateTruncate TruncateTo)
    {
        if (TruncateTo == DateTruncate.Year)
            return new DateTime(dt.Year, 0, 0);
        else if (TruncateTo == DateTruncate.Month)
            return new DateTime(dt.Year, dt.Month, 0);
        else if (TruncateTo == DateTruncate.Day)
            return new DateTime(dt.Year, dt.Month, dt.Day);
        else if (TruncateTo == DateTruncate.Hour)
            return new DateTime(dt.Year, dt.Month, dt.Day, dt.Hour, 0, 0);
        else if (TruncateTo == DateTruncate.Minute)
            return new DateTime(dt.Year, dt.Month, dt.Day, dt.Hour, dt.Minute, 0);
        else 
            return new DateTime(dt.Year, dt.Month, dt.Day, dt.Hour, dt.Minute, dt.Second);

    }
    public enum DateTruncate
    {
        Year,
        Month,
        Day,
        Hour,
        Minute,
        Second
    }

With Peter Ivan's solution, it is natural to just do `public static DateTime TruncateTo(this DateTime dt, long truncateResolution) { return dt.AddTicks(-(dt.Ticks % truncateResolution)); }`. Much shorter and clearer. Then use like this: `DateTime dtSecs = DateTime.Now.TruncateTo(TimeSpan.TicksPerSecond); DateTime dtHrs = DateTime.Now.TruncateTo(TimeSpan.TicksPerHour);` and so on. You can even truncate to more strange resolutions, like multiples of 3 hours (`3 * TimeSpan.TicksPerHour`). — Jeppe Stig Nielsen, Jul 28 '16 at 11:32

score 5 · Answer 7 · answered Jul 31 '19 at 09:56

Here is the simplest way of doing this. You can control precision as you want.

bool AreEqual(DateTime a, DateTime b, TimeSpan precision)
{
    return Math.Abs((a - b).TotalMilliseconds) < precision.TotalMilliseconds;
}

and usage is pretty self-explanatory

var _ = AreEqual(a, b, precision: TimeSpan.FromSeconds(1));

score 3 · Answer 8 · answered Mar 08 '18 at 09:57

instead of trimming unrelevant DateTime parts via creating new DateTimes, compare only relevant parts:

public static class Extensions
{
    public static bool CompareWith(this DateTime dt1, DateTime dt2)
    {
        return
            dt1.Second == dt2.Second && // 1 of 60 match chance
            dt1.Minute == dt2.Minute && // 1 of 60 chance
            dt1.Day == dt2.Day &&       // 1 of 28-31 chance
            dt1.Hour == dt2.Hour &&     // 1 of 24 chance
            dt1.Month == dt2.Month &&   // 1 of 12 chance
            dt1.Year == dt2.Year;       // depends on dataset
    }
}

I took answer by Dean Chalk as base for performance comparison, and results are:

CompareWith is a bit faster than TrimMilliseconds in case of equal dates
CompareWith is a faster than dates are not equal

my perf test (run in Console project)

static void Main(string[] args)
{
    var dtOrig = new DateTime(2018, 03, 1, 10, 10, 10);
    var dtNew = dtOrig.AddMilliseconds(100);

    //// perf run for not-equal dates comparison
    //dtNew = dtNew.AddDays(1);
    //dtNew = dtNew.AddMinutes(1);

    int N = 1000000;

    bool isEqual = false;

    var sw = Stopwatch.StartNew();
    for (int i = 0; i < N; i++)
    {
        // TrimMilliseconds comes from 
        // https://stackoverflow.com/a/7029046/1506454 
        // answer by Dean Chalk
        isEqual = dtOrig.TrimMilliseconds() == dtNew.TrimMilliseconds();
    }
    var ms = sw.ElapsedMilliseconds;
    Console.WriteLine("DateTime trim: " + ms + " ms");

    sw = Stopwatch.StartNew();
    for (int i = 0; i < N; i++)
    {
        isEqual = dtOrig.CompareWith(dtNew);
    }
    ms = sw.ElapsedMilliseconds;
    Console.WriteLine("DateTime partial compare: " + ms + " ms");

    Console.ReadKey();
}

score 3 · Answer 9 · answered Aug 11 '11 at 15:53

3

One way would be to create new dates, inputting the year, month, day, hour, minute, second into the constructor. Alternatively, you could simply compare each value separately.

answered Aug 11 '11 at 15:53

drharris

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score 2 · Answer 10 · answered Aug 11 '11 at 15:54

2

Ether set the milliseconds in your other datetime to zero, or subtract one date from the other and just check the TotalMinutes property of the resulting time span.

answered Aug 11 '11 at 15:54

Jonathan Wood

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Ahmed Fekry · Answer 11 · 2021-01-17T11:22:43.210

Simply you can use datetime format with the format you want, and convert it again to datetime as below,

//compare file's dates
        
        String format1 = @"yyyy-MM-dd HH:mm:ss"; // you also can avoid seconds if you want

        public bool CompareByModifiedDate(string strOrigFile, string strDownloadedFile)
        {
            
            //.here we will use the format

            DateTime dtOrig = Convert.ToDateTime(File.GetLastWriteTime(strOrigFile).ToString(format1));
            DateTime dtNew = Convert.ToDateTime(File.GetLastWriteTime(strDownloadedFile).ToString(format1));

            if (dtOrig == dtNew)
                return true;
            else
                return false;
        }

Bobby Borszich · Answer 12 · 2011-08-11T16:11:37.347

1

You could create an extension method that would set the milliseconds to zero for a DateTime object

public static DateTime ZeroMilliseconds(this DateTime value) {
  return new DateTime(value.Year, value.Month, value.Day, 
    value.Hours, value.Minutes, value.Seconds);
}

Then in your function

 if (dtOrig.ZeroMilliseconds() == dtNew.ZeroMilliseconds())
        return true;
     else
        return false;

edited Aug 11 '11 at 16:11

answered Aug 11 '11 at 15:56

Bobby Borszich

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So your method returns a DateTime but is defined as returning a string. – Ash Burlaczenko Aug 11 '11 at 15:59
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I don't know why all programmers needs too many lines when rteurning a bool function with – Mickey Mouse Oct 14 '14 at 14:21
The value.Kind should be set in new DateTime – Doug Domeny Jun 09 '20 at 14:03

score -1 · Answer 13 · answered Oct 14 '16 at 16:45

-1

cast sortable strings and compare. simple and run well.

    return string.Compare(dtOrig.ToString("s"), dtNew.ToString("s"), 
StringComparison.Ordinal) == 0;

answered Oct 14 '16 at 16:45

Nuri YILMAZ

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M. Smith · Answer 14 · 2016-01-13T23:27:26.187

-3

The most straightforward way to truncate time is to format it and parse on the units that you want:

var myDate = DateTime.Parse(DateTime.Now.ToString("MM/dd/yyyy hh:mm:ss"));

DOK's method re-written

public bool CompareByModifiedDate(string strOrigFile, string strDownloadedFile)
    {
         DateTime dtOrig = DateTime.Parse(File.GetLastWriteTime(strOrigFile).ToString("MM/dd/yyyy hh:mm:ss"));
         DateTime dtNew = DateTime.Parse(File.GetLastWriteTime(strDownloadedFile).ToString("MM/dd/yyyy hh:mm:ss"));

         if (dtOrig == dtNew)
            return true;
         else
            return false;
    }

edited Jan 13 '16 at 23:27

answered Jan 13 '16 at 23:22

M. Smith

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Run your code in the UK, Europe or Australia and see what happens. I'm not a fan of this approact anyway, but you need to be using an invariant culture or `DateTime.ParseExact` for this to be even remotely reliable. – Simon MᶜKenzie Jan 14 '16 at 00:02

Mickey Mouse · Answer 15 · 2015-05-20T20:12:28.743

-6

Don't know why almost all programmers needs extra lines to return a bool value from a function with a bool expression.

instead

if (dtOrig.ZeroMilliseconds() == dtNew.ZeroMilliseconds())
    return true;
 else
    return false;

you can always just use

return dtOrig.ZeroMilliseconds() == dtNew.ZeroMilliseconds()

if the expression is true it returns true else false.

edited May 20 '15 at 20:12

answered Oct 14 '14 at 14:27

Mickey Mouse

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where did you get ZeroMilliseconds() from? – Purusartha Oct 30 '14 at 23:36

Ignore milliseconds when comparing two datetimes

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