How do I subtract a localDate from another LocalDate

Question

I am trying to calculate the number of days between 2 localDates

I am taking inspiration from this answer: https://stackoverflow.com/a/325964 to this question Determine Whether Two Date Ranges Overlap

The trouble is, this question uses dateTime which I believe is outdated, hence the reason for using localDate.

Does anyone know of a similar way to implement the algorithm in the above answer using localDate instead.

The minus method doesn't allow subtraction of another localDate.

I have tried using

ChoronoUnit.DAYS.between(LD1, LD2.plusDays(1)) //include the final day in the count

but there are occasions when this produces a negative number so the algorithm breaks because it chooses the smallest number as the number of days overlap

The code example seems to be correct. What exactly do you mean by "algorithm falls over"? Your question is a little bit unclear :-) — Tobias, Apr 29 '20 at 12:07
If `ChronoUnit.DAYS.between(ld1, ld2)` returns a negative number then `ld1.isAfter(ld2)` must return true. You can easily stop getting a negative number by always making the future date be the second argument or, even more simple, just use `days = Math.abs(days)`. — Slaw, Apr 29 '20 at 12:22
It is stated in the [javadoc](https://docs.oracle.com/javase/8/docs/api/java/time/temporal/ChronoUnit.html#between-java.time.temporal.Temporal-java.time.temporal.Temporal-) of method `between()` in `ChronoUnit` as follows: _The result will be negative if the end is before the start._ — Abra, Apr 29 '20 at 12:34
@Abra I have read that but the thing that confuses me is that in the algorithm here the returned answer is always positive but I can't work out what I have done wrong with my algorithm — 3rdRockSoftware, Apr 29 '20 at 12:40
[edit] your question and post a [mcve] demonstrating how you get a negative result. — Abra, Apr 29 '20 at 12:44
@3rdRockSoftware In the linked answer, to paraphrase, it says: *"If start and end dates can be out of order, ... use `Min()` and `Max()`"*, as in `ChoronoUnit.DAYS.between(Math.min(LD1, LD2), Math.max(LD1, LD2)) + 1`. [Slaw's comment](https://stackoverflow.com/questions/61501388/how-do-i-subtract-a-localdate-from-another-localdate#comment108792949_61501388) is a better solution: `Math.abs(ChoronoUnit.DAYS.between(LD1, LD2)) + 1` — Andreas, Apr 29 '20 at 13:14
@Andreas I had thought of using `abs`. I'll try that and see if I can produce some sensible results — 3rdRockSoftware, Apr 29 '20 at 13:30

score 5 · Accepted Answer · answered May 01 '20 at 19:09

5

Andreas already gave the answer in a comment: you need to take the absolute value of the day difference first, then add 1 to make the count inclusive of both start and end date.

public static long daysBetweenInclusive(LocalDate ld1, LocalDate ld2) {
    return Math.abs(ChronoUnit.DAYS.between(ld1, ld2)) + 1;
}

Let’s try it out:

    LocalDate ld1 = LocalDate.of(2020, Month.MAY, 4);
    System.out.println(daysBetweenInclusive(ld1, LocalDate.of(2020, Month.MAY, 2)));
    System.out.println(daysBetweenInclusive(ld1, ld1));
    System.out.println(daysBetweenInclusive(ld1, LocalDate.of(2020, Month.MAY, 7)));

Output:

3
1
4

answered May 01 '20 at 19:09

Ole V.V.

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What does "then add 1 to make the count inclusive of both start and end date" mean? If I calculate the difference between a day and itself with your algorithm, I would expect to get 0 as the result and not 1. Would it mean I get 1 in the meaning of "1st day"? – kitekat Oct 30 '22 at 16:02
@kitekat Yes, exactly. The question mentioned `include the final day in the count`. So while `ChronoUnit.DAYS.between()` takes the days from Tuesday to Thursday in the same week to be 2 (Tue and Wed), the OP wanted to include Thursday in the count to get 3. So I believe the OP also wanted the count from a day to itself to be 1, namely that day. If your requirement differs, just do not add 1. – Ole V.V. Oct 30 '22 at 18:19
@oleV.V. Yes that is correct. If the start day is Monday and the end day is Monday, then I needed count = 1 – 3rdRockSoftware Jul 31 '23 at 11:31

How do I subtract a localDate from another LocalDate

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