def to_s
      return "" if @seconds.nil?

      hours, seconds = @seconds.divmod(Hour.to_seconds(1))
      minutes = seconds / Minute.to_seconds(1)
      sprintf("%d:%02d", hours, minutes)
    end
def to_s
      return "" if @seconds.nil?

      hours, seconds = @seconds.divmod(Hour.to_seconds(1))
      minutes = seconds / Minute.to_seconds(1)
      sprintf("%d:%02d", hours, minutes)
    end
Returns the duration in hours and minutes format: "hh:mm"