Code koan: Dividing a `DateInterval` into sub-intervals by `Calendar.Component`s in Swift 4

Calendar+Divide.swift

extension Calendar {
    func divide(interval dividend: DateInterval, by component: Calendar.Component, value divisor: Int) -> [DateInterval]? {
        var intervals: [DateInterval] = []

        var previousDate = dividend.start
        while
            let nextDate = self.date(byAdding: component, value: divisor, to: previousDate),
            dividend.contains(nextDate)
        {
            let interval = DateInterval(start: previousDate, end: nextDate)
            intervals.append(interval)
            previousDate = nextDate
        }

        return intervals
    }
}

The initial implementation ignores the remainder for DateIntervals that cannot be evenly divided.