extension NSCalendar {

    private struct Keys {
        static let calendarKey = "SharedGregorianCalendar"
    }

    class var sharedGregorianCalendar: NSCalendar {
        let threadDictionary = NSThread.currentThread().threadDictionary
        if let calendar = threadDictionary[Keys.calendarKey] as? NSCalendar {
            return calendar
        } else {
            let calendar = NSCalendar(calendarIdentifier: NSGregorianCalendar)!
            threadDictionary[Keys.calendarKey] = calendar
            return calendar
        }
    }

}