code: public void add(TemporalAmount amountToAdd) {
          Objects.requireNonNull(amountToAdd, "amountToAdd");
          synchronized (instantHolder) {
              ZonedDateTime current = ZonedDateTime.ofInstant(instantHolder.get(), zone);
              ZonedDateTime result = current.plus(amountToAdd);
              instantHolder.set(result.toInstant());
          }
      }
docs: Adds the specified amount to this clock.
  <p>
  Atomically updates this clock to the value of the following expression:
  <pre>
  ZonedDateTime.now(thisClock)
  .plus(amountToAdd)
  .toInstant()
  </pre>
  
  @param amountToAdd the amount to add, not null
  @throws DateTimeException if the addition cannot be made
  @throws ArithmeticException if numeric overflow occurs
  @see ZonedDateTime#plus(TemporalAmount)