type countable interface {
	HoursWorked() time.Duration
}

type WorkDay struct {
	StartTime time.Time
	EndTime   time.Time
}

type WorkWeek struct {
	WorkDays []WorkDay
}

type WorkMonth struct {
	WorkWeeks []WorkWeek
}

func (w WorkDay) HoursWorked() time.Duration {
	// Find hours worked through simple subtraction.
}

func (w WorkWeek) HoursWorked() time.Duration {
	return sumHoursWorked(w.WorkDays)
}

func (w WorkMonth) HoursWorked() time.Duration {
	return sumHoursWorked(w.WorkWeeks)
}

func sumHoursWorked(timeFrames []countable) time.Duration {
	var totalHours time.Duration
	for i := range timeFrames {
		totalHours += timeFrames[i].HoursWorked()
	}
	return totalHours
}