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
    }()
    
}

better: ISO8601DateFormatter

better: ISO8601DateFormatter

Do you know how to convert an ISO8601DateFormatter into DateFormatter?
ISO8601DateFormatter inherits from Formatter wich is not compliant to DateFormatter, so I cannot set the ISO Formatter into
encoder.dateEncodingStrategy for Decoding & Encoding JSONs

better: ISO8601DateFormatter

Yes, this is an old GIST :)

better: ISO8601DateFormatter

Do you know how to convert an ISO8601DateFormatter into DateFormatter?
ISO8601DateFormatter inherits from Formatter wich is not compliant to DateFormatter, so I cannot set the ISO Formatter into
encoder.dateEncodingStrategy for Decoding & Encoding JSONs

Actually this is already built in:

decoder.dateEncodingStrategy = .iso8601

Also if you need to customise the formatter you can do it via .custom

encoder.dateEncodingStrategy = .custom { date, encoder in
    var container = encoder.singleValueContainer()
    let formatter = ISO8601DateFormatter()
    formatter.formatOptions = .withColonSeparatorInTime
    try container.encode(formatter.string(from: date))
}