code: public void add(PGInterval interval) {
      interval.setYears(interval.getYears() + getYears());
      interval.setMonths(interval.getMonths() + getMonths());
      interval.setDays(interval.getDays() + getDays());
      interval.setHours(interval.getHours() + getHours());
      interval.setMinutes(interval.getMinutes() + getMinutes());
      interval.setSeconds(interval.getSeconds() + getSeconds());
    }
docs: Add this interval's value to the passed interval. This is backwards to what I would expect, but
  this makes it match the other existing add methods.
  
  @param interval intval to add