code: public Timespan add(Timespan other)
    {
      if(getTimeUnit() == other.getTimeUnit())
        return new Timespan(getDuration() + other.getDuration(), getTimeUnit());
  
      return new Timespan(getDurationInMilliseconds() + other.getDurationInMilliseconds(),
                          TimeUnit.MILLISECOND);
    }
docs: Adds another timespan to this timespan and return a brand new one. Note that the unit is
  preserved if <code>other</code> has the same unit as 'this'.
  
  @param other the timespan to add
  @return a brand new timespan.