閏年

scala> :paste
// Entering paste mode (ctrl-D to finish)

  val date = new LocalDate
  val date_1 = date.minusYears(1).plusDays(2)
  val date_2 = date.plusDays(2).minusYears(1)

// Exiting paste mode, now interpreting.

date: org.joda.time.LocalDate = 2020-02-27
date_1: org.joda.time.LocalDate = 2019-03-01
date_2: org.joda.time.LocalDate = 2019-02-28

2020/02/27 の1年前 -> 2019/2/27 の2日後 -> 2019/3/1
2020/02/27 の2日後 -> 2020/2/29 の1年前 -> 2019/2/28

LocalDate#minusYears

    /**
     * Returns a copy of this date minus the specified number of years.
     * <p>
     * This subtracts the specified number of years from the date.
     * If subtracting years makes the day-of-month invalid, it is adjusted to the last valid day in the month.
     * This LocalDate instance is immutable and unaffected by this method call.
     * <p>
     * The following three lines are identical in effect:
     * <pre>
     * LocalDate subtracted = dt.minusYears(6);
     * LocalDate subtracted = dt.minus(Period.years(6));
     * LocalDate subtracted = dt.withFieldAdded(DurationFieldType.years(), -6);
     * </pre>
     *
     * @param years  the amount of years to subtract, may be negative
     * @return the new LocalDate minus the increased years
     */
    public LocalDate minusYears(int years) {
        if (years == 0) {
            return this;
        }
        long instant = getChronology().years().subtract(getLocalMillis(), years);
        return withLocalMillis(instant);
    }

うん

If subtracting years makes the day-of-month invalid, it is adjusted to the last valid day in the month.

minusMonthsもしかり
つまり

scala> :paste
// Entering paste mode (ctrl-D to finish)

    import org.joda.time.LocalDate

    val date = new LocalDate(2019,12,31)
    val date_1 = date.minusMonths(12)
    val date_2 = date.minusMonths(1).minusMonths(1).minusMonths(1).minusMonths(1).minusMonths(1).minusMonths(1).minusMonths(1).minusMonths(1).minusMonths(1).minusMonths(1).
minusMonths(1).minusMonths(1)

// Exiting paste mode, now interpreting.

import org.joda.time.LocalDate
date: org.joda.time.LocalDate = 2019-12-31
date_1: org.joda.time.LocalDate = 2018-12-31
date_2: org.joda.time.LocalDate = 2018-12-28