How to avoid negative time between time difference?

Question

I'm developing an app in which I'm using Java8 Time. I'm facing an issue.

Let's say Time A is 08:00 and Time B is 17:00, so the difference of between these two times will be 9h which in my case is correct, but if Time A is 18:00 and Time B is 02:00 it should be 8h, but in my case my program is returning -16. Kindly someone guide me how to solve this.

My code:

@Test
public void testTime()
{
    DateTimeFormatter format = DateTimeFormatter.ofPattern("HH:mm");

    String s = "18:00";
    String e = "02:00";

    // Parse datetime string to java.time.LocalDateTime instance
    LocalTime startTime = LocalTime.parse(s, format);
    LocalTime endTime = LocalTime.parse(e, format);

    String calculatedTime = ChronoUnit.HOURS.between(startTime, endTime)%24 + ":"
            + ChronoUnit.MINUTES.between(startTime, endTime)%60;

    System.out.println(calculatedTime);

}

The difference computation is correct, actually, I would say it is the problem of how you are using it. There is very little value in working with time on its own, and for just `LocalTime` it is correct to say that there is -16 hours between 18:00 and 2:00. What you need is an instance of `LocalDateTime` with properly set `LocalDate`, and then you can have your correct computation. — M. Prokhorov, Jun 06 '17 at 10:39
@Bathsheba, mine is not an answer yet, it is just a comment. — M. Prokhorov, Jun 06 '17 at 10:40
@ΦXocę웃Пepeúpaツ No he's not -- ChronoUnit.HOURS.between(startTime, endTime) returns a number, and so does ChronoUnit.MINUTES.between(startTime, endTime). But he can figure out for himself how to apply Math.abs correctly, not going to spoon feed in comments. — Erwin Bolwidt, Jun 06 '17 at 10:41
@ErwinBolwidt Math.abs is the wrong thing to use. He needs modulo 24 arithmetic with result between 0 and 23. Example: time elapsed from 23:00 to 02:00 is either -21 hours or 3 hours, but certainly not 21 hours. — Klitos Kyriacou, Jun 06 '17 at 10:53

score 11 · Accepted Answer · answered Jun 06 '17 at 11:40

Why not use the Duration class? It’s meant for situations like yours.

    Duration calculatedTime = Duration.between(startTime, endTime);
    if (calculatedTime.isNegative()) {
        calculatedTime = calculatedTime.plusDays(1);
    }

    System.out.println(calculatedTime);

This prints the duration in ISO 8601 format:

PT8H

To format it in Java 8:

    long hours = calculatedTime.toHours();
    calculatedTime = calculatedTime.minusHours(hours);
    String formattedTime = String.format(Locale.getDefault(), "%d:%02d",
                                         hours, calculatedTime.toMinutes());
    System.out.println(formattedTime);

This prints

8:00

To format in Java 9 (not tested):

    String formattedTime = String.format("%d:%02d", 
                                         calculatedTime.toHoursPart(),
                                         calculatedTime.toMinutesPart());

Bathsheba · Answer 2 · 2017-06-06T10:39:04.337

5

If the time difference comes out negative, then add 24.

Simples!

(Obviously this will ignore overnight changes to daylight saving time, so your best bet really is to do this properly using time duration objects. That said though, you'll then need to know more about the dates associated with the two times.)

edited Jun 06 '17 at 10:39

answered Jun 06 '17 at 10:35

Bathsheba

231,907
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361
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@ΦXocę웃Пepeúpaツ Read: *"If the time difference comes out negative"*. Neither is 02:00 negative, nor the difference in OPs example. – Tom Jun 06 '17 at 10:47

ΦXocę 웃 Пepeúpa ツ · Answer 3 · 2017-06-06T10:45:43.957

check if the startTime is after endTime and swap the opernads if required:

    LocalTime startTime = LocalTime.parse(s, format);
    LocalTime endTime = LocalTime.parse(e, format);
    String calculatedTimeMorning;

    if (startTime.isAfter(endTime)) {
        calculatedTimeMorning = (ChronoUnit.HOURS.between(endTime, startTime) % 24) + ":"
                + (ChronoUnit.MINUTES.between(endTime, startTime) % 60);
    } else {
        calculatedTimeMorning = (ChronoUnit.HOURS.between(startTime, endTime) % 24) + ":"
                + (ChronoUnit.MINUTES.between(startTime, endTime) % 60);
    }

score 1 · Answer 4 · answered Jun 06 '17 at 10:41

1

You have to calculate time differences modulo 24. So take the difference as you already do and then compute it modulo 24. Note that in Java -16%24 would return -16 so what you really need to do is the following:

public static int modulus(int n, int m){
    return  (n < 0) ? (m - (Math.abs(n) % m) ) %m : (n % m);
}

which always return a positive result.

answered Jun 06 '17 at 10:41

Davide Spataro

7,319
1
24
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1

U wot? Why not add 24 and compute the result modulo 24? – Bathsheba Jun 06 '17 at 10:48
in this case adding 24 works fine. But that function is more general. Imagine having a number like `-251`, adding 24 would not yield to a positive number. – Davide Spataro Jun 06 '17 at 10:53

How to avoid negative time between time difference?

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