Time difference calculation bug

Roger Riggs roger.riggs at oracle.com
Thu May 4 16:10:19 UTC 2023 

Hi,

I created a Jira issue:
8307466 <https://bugs.openjdk.org/browse/JDK-8307466> java.time.Instant 
Time difference calculation bug

The root cause of the bug is in the cod that manages the representation 
of Instant as seconds and nanoseconds.
The borrow/carry logic between the seconds and nanoseconds is suspect in 
Instant.until(Instant end, ChronoUnit unit).

Thanks for the bug report, Roger


On 5/3/23 4:47 PM, SHARPE, Stuart (Commercial & Institutional CDIO) wrote:
> Hi Roger,
>
> Thanks for the reply.
>
> I must admit I am struggling to understand your explanation. I can't see how the choice of internal representation would affect the result in this way. I also don't see anything in the JavaDoc that describes values being truncated prior to the calculation, and even if it did, the results are not consistent - if that was the expected behaviour, the results from my previous example would have been 1000 and 1001, would they not?
>
> The part I quoted from the JavaDoc seems quite clear: it should return the number of whole units (millis in this case) between the two temporals. The number of milliseconds between 10:00:00.000999 and 10:00:01.000 is 999.001, which is 999 whole units.
>
> I think the below illustration makes it clearer that there is something wrong. The output values are all what I would expect as per the JavaDoc apart from the result for t2 = 10:00:01.000.
>
>      var t1 = Instant.parse("2023-05-03T10:00:00.000999Z");
>      var t2 = Instant.parse("2023-05-03T10:00:00.995Z");
>      
>      for(int i=0; i<10; i++) {
>        System.out.println(ChronoUnit.MILLIS.between(t1, t2));
>        t2 = t2.plusMillis(1);
>      }
>
> This prints:
>
> 994
> 995
> 996
> 997
> 998
> 1000
> 1000
> 1001
> 1002
> 1003
>
> I'm fully prepared to accept that I've missed something, but at the moment I can't understand what that is.
>
> Regards,
>
> Stuart
>
>
> From: core-libs-dev<core-libs-dev-retn at openjdk.org>  On Behalf Of Roger Riggs
> Sent: 03 May 2023 20:58
> To:core-libs-dev at openjdk.org
> Subject: Re: Time difference calculation bug
>
>
> Hi,
>
> The seconds and nano-seconds are computed separately.
> The representation of Instant holds seconds and nanoseconds separately.
>
> The computation is performed on the nano-seconds of the Instant and truncated to milliseconds.
> The value of 0.000999 becomes 0 when truncated to MILLIS.
>
> Regards, Roger
> On 5/3/23 12:54 PM, SHARPE, Stuart (Commercial & Institutional CDIO) wrote:
> Hello,
>   
> I believe I have discovered a bug in java.time. I've searched Jira and couldn't find any existing similar issue.
>   
> Consider the following code:
>   
>      var t1 = Instant.parse("2023-05-03T10:00:00.000999Z");
>      var t2 = Instant.parse("2023-05-03T10:00:01.000Z");
>      var t3 = Instant.parse("2023-05-03T10:00:01.001Z");
>      
>      System.out.println(MILLIS.between(t1, t2));
>      System.out.println(MILLIS.between(t1, t3));
>   
> This prints:
>   
>      1000
>      1000
>   
> Given that t3 is exactly one millisecond later than t2, it does not seem reasonable that they can both be 1000 milliseconds later than t1.
>   
> The JavaDoc for between() states "The calculation returns a whole number, representing the number of complete units between the two temporals."
>   
> Assuming I have understood this correctly, I think MILLIS.between(t1, t3) is correct but MILLIS.between(t1, t2) should return 999.
>   
> Regards
>   
> Stuart Sharpe
>   
>
>
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