Period between LocalDates produces wrong results

I see that java.time.Period with java.time.LocalDates works mostly as desired and I can use it to get the duration between two dates in years, months and days but I found an example that is not working (not as I expected it), like following:

val test1 = LocalDate.of(2023, 1, 31)
val test2 = LocalDate.of(2023, 1, 28)
val testToday = LocalDate.of(2023, 3, 9)

val period1 = Period.between(test1, testToday)
val period2 = Period.between(test2, testToday)

L.d { "test1 = $test1 | test2 = $test2 | today = $testToday" }
L.d { "period1 = $period1 | period2 = $period2" }

Output is following:

period1 = P1M9D | period2 = P1M9D
test1 = 2023-01-31 | test2 = 2023-01-28 | today = 2023-03-09

Problem

The solution above tells me following:

• Duration between 2023-01-31 and 2023-03-09 => 1 month and 9 days (P1M9D)
• Duration between 2023-01-28 and 2023-03-09 => 1 month and 9 days (P1M9D)

=> but this is wrong, the second one should show 3 days more as duration... Is there an explanation for this?

The "problem" is the way Period.between performs its calculation:

> The start date is included, but the end date is not. The period is
> calculated by removing complete months, then calculating the remaining
> number of days, adjusting to ensure that both have the same sign. The
> number of months is then split into years and months based on a 12
> month year. A month is considered if the end day-of-month is greater
> than or equal to the start day-of-month. For example, from 2010-01-15
> to 2011-03-18 is one year, two months and three days.

This results in 2023-01-28 until 2023-01-01 counting as 1 month and 1 day (P1M1D), but so will 2023-01-29, 30, and 31. This is because 2023-01-31 plus 1 month is 2023-02-31, which is invalid, so it uses 2023-02-28 (and same for 29 and 30, and of course 28). In other words, there is 1 full month, which ends on 2023-02-28, and then there are 9 more days until 2023-03-09 (exclusive).

This conforms to the behaviour documented for LocalDate.plusMonths:

> This method adds the specified amount to the months field in three
> steps:
>
> 1. Add the input months to the month-of-year field
> 2. Check if the resulting date would be invalid
> 3. Adjust the day-of-month to the last valid day if necessary
>
> For example, 2007-03-31 plus one month would result in the invalid
> date 2007-04-31. Instead of returning an invalid result, the last
> valid day of the month, 2007-04-30, is selected instead.