Date ranges with NSCalendar in Swift

PrintDates.swift

print("Day test = \(rangeOfPeriod(.Day, date: NSDate()))")
print("Week test = \(rangeOfPeriod(.WeekOfYear, date: NSDate()))")
print("month test = \(rangeOfPeriod(.Month, date: NSDate()))")
print("Year test = \(rangeOfPeriod(.Year, date: NSDate()))")

RangeOfPeriod.swift

func rangeOfPeriod(period: NSCalendarUnit, date: NSDate) -> (NSDate, NSDate) {
        let calendar = NSCalendar.currentCalendar()
        var startDate: NSDate? = nil
        var duration: NSTimeInterval = 0

        calendar.rangeOfUnit(period, startDate: &startDate, interval: &duration, forDate: date)

        let endDate = startDate!.dateByAddingTimeInterval(duration - 1)

        return (startDate!, endDate)
}