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How to use ISO-8601 dates with JSONDecoder and Codable

Written by Paul Hudson    @twostraws

Encoding and decoding dates using the Codable protocol isn’t hard to do, but it does produce some unexpected data by default: Date stores its information as a floating-point number counting the number of seconds since January 1st 2001, rather than something standard involved days, months, and years.

ISO-8601 is the web’s standard way of referring to dates and times, and looks like this: 2018-12-25T17:30:00Z – in English that’s December 25th 2018, at 5:30pm UTC. Both JSONEncoder and JSONDecoder are able to use this date format rather than the floating-point default – all you have to do is set their dateEncodingStrategy and dateDecodingStrategy properties.

If you’re decoding dates, use this:

let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .iso8601

And if you’re encoding dates you should use this:

let encoder = JSONEncoder()
encoder.dateEncodingStrategy = .iso8601

Available from iOS 8.0

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