LocalDate.plus Incorrect Answer

To complement assylias' answer, here is a simplified example to show why this happens:

    public static void main(String[] args)
    {
        LocalDate a = LocalDate.of(1992, Month.APRIL, 1);
        LocalDate b = LocalDate.of(1992, Month.MAY, 1);
        // Calculate the period. It will return "One month"
        Period period = Period.between(a, b);
        // Add one month to b. It will return June 1, 1992
        LocalDate c = b.plus(period);
        System.out.println(ChronoUnit.DAYS.between(a, b)); // 30 days as April has 30 days
        System.out.println(ChronoUnit.DAYS.between(b, c)); // 31 days as May has 31 days
    }

A Period is made of a number of years, months and days. In your case, Period.between(birthA, eventA) is 26 years, 7 months and 12 days.

If you add that to birthA, you get:

• 1965 + 26 years -> 1991
• September 1991 + 7 months -> April 1991
• April 27, 1991 + 12 days -> May 9, 1992

Which works as expected.

If you apply the same calculation, starting from May 9, 1992, you get December 21, 2018.

If you want to add a certain number of days instead, you can't simply add the period (as years and months don't always have the same length). One option is to use ChonoUnit.DAYS.between instead:

LocalDate halfA = eventA.plusDays(ChronoUnit.DAYS.between(birthA, eventA));

That returns 2018-12-20 which I think is what you expected.