Method plus

base/src/main/java/net/time4j/PlainTimestamp.java:1260–1279

(
        long amount,
        CalendarUnit unit
    )

     * @since   4.19
     */
    public PlainTimestamp plus(
        long amount,
        CalendarUnit unit
    ) {

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

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

    }

    /**
     * <p>Adds given amount in units to this timestamp and yields the result of addition. </p>