code: public boolean isBefore(Day day)
    {
      if (day == null)
        throw new IllegalArgumentException("day cannot be null");
  
      return localDate.isBefore(day.localDate);
    }
docs: Return true if this day is before the specified day.
  
  @param day  Day to compare to.
  @return  True if this is before day, false otherwise.
  @throws IllegalArgumentException  If day is null.