iso8601 date json decoding using swift4

TL;DR version: it only parses the withInternetDateTime format of the ISO8601DateFormatter described here. This means that your string should not have milliseconds.

More info:

Looking at the Swift source on line 787, the comment says:

/// Decode the `Date` as an ISO-8601-formatted string (in RFC 3339 format).

Looking at that RFC, it gives a couple of (admittedly tricky) examples in section 5.8:

1985-04-12T23:20:50.52Z
1996-12-19T16:39:57-08:00
1996-12-20T00:39:57Z
1990-12-31T23:59:60Z
1990-12-31T15:59:60-08:00
1937-01-01T12:00:27.87+00:20

Only the second and the third example are actually decoded by Swift, the rest fails. It seems to me that either the comment is incorrect, or the implementation is not complete. As for the real implementation, that's outside the Swift source, it simply seems to use the ISO8601DateFormatter class in Foundation.

The Swift unittest is also very limited, see line 180. It simply encodes a single date, and then decodes it back. So in other words, the only thing that's tested, is the format that the ISO8601DateFormatter outputs by default, which is hardcoded to the option .withInternetDateTime, described here.

You can use like this :

enum DateError: String, Error {
    case invalidDate
}

let decoder = JSONDecoder() 

let formatter = DateFormatter()
formatter.calendar = Calendar(identifier: .iso8601)
formatter.locale = Locale(identifier: "en_US_POSIX")
formatter.timeZone = TimeZone(secondsFromGMT: 0)

decoder.dateDecodingStrategy = .custom({ (decoder) -> Date in
    let container = try decoder.singleValueContainer()
    let dateStr = try container.decode(String.self)

    formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSXXXXX"
    if let date = formatter.date(from: dateStr) {
        return date
    }
    formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ssXXXXX"
    if let date = formatter.date(from: dateStr) {
        return date
    }
    throw DateError.invalidDate
})