Apple docs state that setting dateDecodingStrategy to .iso8601 will decode the ISO-8601-formatted string (in RFC 3339 format) to Date object. But most of the example date formats from https://www.ietf.org/rfc/rfc3339 fail to parse.

Decoder.swift

import UIKit

struct Event: Codable {
    var date: Date
}

/*
 Apple docs state that setting dateDecodingStrategy to .iso8601 will decode the ISO-8601-formatted string (in RFC 3339 format) to Date object.
 But most of the example date formats from https://www.ietf.org/rfc/rfc3339 fail to parse.
 */

let dates = ["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"]

for date in dates {
    let json = "{ \"date\": \"" + date + "\" }"
    guard let data = json.data(using: .utf8) else { break }
    
    let decoder = JSONDecoder()
    decoder.dateDecodingStrategy = .iso8601
    
    if let event = try? decoder.decode(Event.self, from: data) {
        print("\(date) ✅ decoded as \(event.date)")
    } else {
        print("\(date) ❌")
    }
}

Output

1985-04-12T23:20:50.52Z ❌
1996-12-19T16:39:57-08:00 ✅ decoded as 1996-12-20 00:39:57 +0000
1996-12-20T00:39:57Z ✅ decoded as 1996-12-20 00:39:57 +0000
1990-12-31T23:59:60Z ❌
1990-12-31T15:59:60-08:00 ❌
1937-01-01T12:00:27.87+00:20 ❌