code: public Duration subtract( long duration,
                                TimeUnit unit ) {
          long durationInNanos = TimeUnit.NANOSECONDS.convert(duration, unit);
          return new Duration(this.durationInNanos - durationInNanos);
      }
docs: Subtract the supplied duration from this duration, and return the result.
  
  @param duration the duration to subtract from this object
  @param unit the unit of the duration being subtracted; may not be null
  @return the total duration