Method plus

base/src/main/java/net/time4j/PlainDate.java:1384–1403

(
        long amount,
        CalendarUnit unit
    )

     * @since   4.18
     */
    public PlainDate plus(
        long amount,
        CalendarUnit unit
    ) {

        if (unit == null) {
            throw new NullPointerException("Missing unit.");
        } else if (amount == 0) {
            return this;
        }

        try {
            return PlainDate.doAdd(unit, this, amount, OverflowUnit.POLICY_PREVIOUS_VALID_DATE);
        } catch (IllegalArgumentException iae) {
            ArithmeticException ex = new ArithmeticException("Result beyond boundaries of time axis.");
            ex.initCause(iae);
            throw ex;
        }

    }

    /**
     * <p>Subtracts given amount in units from this date and yields the result of subtraction. </p>