DateFormatter+ISO8601.swift

// Swift 3
extension DateFormatter {
    
    public static var iso8601: DateFormatter {
        return DateFormatter.iso8601DateFormatter
    }
    
    private static let iso8601DateFormatter: DateFormatter = {
        let formatter = DateFormatter()
        formatter.locale = Locale(identifier: "en_US_POSIX")
        formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ssZZZZZ"
        return formatter
    }()
    
}

NSDateFormatter+ISO8601.swift

// Swift 2.2
extension NSDateFormatter {
    
    public static var iso8601: NSDateFormatter {
        return NSDateFormatter.iso8601DateFormatter
    }
    
    private static let iso8601DateFormatter: NSDateFormatter = {
        let formatter = NSDateFormatter()
        formatter.locale = NSLocale(localeIdentifier: "en_US_POSIX")
        formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ssZZZZZ"
        return formatter
    }()
    
}