code: public Duration plus(long amount, TimeUnit timeUnit) {
          if (timeUnit == null) {
              throw new IllegalArgumentException("Time unit cannot be null");
          }
  
          return new Plus().apply(this, new Duration(amount, timeUnit));
      }
docs: Add a amount with the given time unit. For example:
  <pre>
  new Duration(2, SECONDS).plus(4, MILLISECONDS)
  </pre>
  will return a Duration of 2004 {@link TimeUnit#MILLISECONDS}.
  <p>
  <pre>
  new Duration(2, SECONDS).plus(1, MINUTES)
  </pre>
  will return a Duration of 62 {@link TimeUnit#SECONDS}.
  
  @param amount   The amount to add
  @param timeUnit The time unit to add
  @return A new amount