Method plus

base/src/main/java/net/time4j/Moment.java:1367–1433

(
        long amount,
        SI unit
    )

     * @throws  ArithmeticException in case of overflow
     */
    public Moment plus(
        long amount,
        SI unit
    ) {

        Moment.check1972(this);

        if (amount == 0) {
            return this;
        }

        Moment result;

        try {
            switch (unit) {
                case SECONDS:
                    if (LeapSeconds.getInstance().isEnabled()) {
                        result = new Moment(
                            Math.addExact(this.getElapsedTimeUTC(), amount),
                            this.getNanosecond(),
                            UTC);
                    } else {
                        result = Moment.of(
                            Math.addExact(this.posixTime, amount),
                            this.getNanosecond(),
                            POSIX
                        );
                    }
                    break;
                case NANOSECONDS:
                    long sum =
                        Math.addExact(this.getNanosecond(), amount);
                    int nano = (int) Math.floorMod(sum, MRD);
                    long second = Math.floorDiv(sum, MRD);

                    if (LeapSeconds.getInstance().isEnabled()) {
                        result = new Moment(
                            Math.addExact(this.getElapsedTimeUTC(), second),
                            nano,
                            UTC
                        );
                    } else {
                        result = Moment.of(
                            Math.addExact(this.posixTime, second),
                            nano,
                            POSIX
                        );
                    }
                    break;
                default:
                    throw new UnsupportedOperationException();
            }
        } catch (IllegalArgumentException iae) {
            ArithmeticException ex =
                new ArithmeticException(
                    "Result beyond boundaries of time axis.");
            ex.initCause(iae);
            throw ex;