def -(other)
      if other.instance_of?(TimeOfDay)
        TimeOfDay.from_second_of_day @second_of_day - other.second_of_day
      else
        TimeOfDay.from_second_of_day @second_of_day - other
      end
    end
def -(other)
      if other.instance_of?(TimeOfDay)
        TimeOfDay.from_second_of_day @second_of_day - other.second_of_day
      else
        TimeOfDay.from_second_of_day @second_of_day - other
      end
    end
Return a new TimeOfDay num_seconds less than self. It will wrap around
 at midnight.