Swift Decodable With Multiple Custom Dates (https://useyourloaf.com/blog/swift-codable-with-custom-dates)

RSSFeed.swift

import Foundation

extension DateFormatter {
    static let iso8601Full: DateFormatter = {
        let formatter = DateFormatter()
        formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSZZZZZ"
        formatter.calendar = Calendar(identifier: .iso8601)
        formatter.timeZone = TimeZone(secondsFromGMT: 0)
        formatter.locale = Locale(identifier: "en_US_POSIX")
        return formatter
    }()

    static let yyyyMMdd: DateFormatter = {
        let formatter = DateFormatter()
        formatter.dateFormat = "yyyy-MM-dd"
        formatter.calendar = Calendar(identifier: .iso8601)
        formatter.timeZone = TimeZone(secondsFromGMT: 0)
        formatter.locale = Locale(identifier: "en_US_POSIX")
        return formatter
    }()
}

public protocol FormattedDate: Codable {
    static var formatter: DateFormatter { get }
    var date: Date { get }
    init(date: Date)
}

extension FormattedDate {

    public init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        let string = try container.decode(String.self)

        guard let date = Self.formatter.date(from: string) else {
            let debug = "Date string does not match format expected by formatter."
            throw DecodingError.dataCorruptedError(in: container, debugDescription: debug)
        }

        self.init(date: date)
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        let string = Self.formatter.string(from: self.date)
        try container.encode(string)
    }
}

struct ISO8601Date: FormattedDate {
    static let formatter: DateFormatter = .iso8601Full
    let date: Date
}

struct YYYMMddDate: FormattedDate {
    static var formatter: DateFormatter = .yyyyMMdd
    let date: Date
}

public struct RSSFeed: Codable {
    public struct Feed: Codable {
        public struct Podcast: Codable {
            public let name: String
            public let artistName: String
            public let url: URL

            private let _releaseDate: YYYMMddDate
            public var releaseDate: Date { return self._releaseDate.date }

            private enum CodingKeys: String, CodingKey {
                case name
                case artistName
                case url
                case _releaseDate = "releaseDate"
            }
        }

        public let title: String
        public let country: String

        private let _updated: ISO8601Date
        public var updated: Date { return self._updated.date }

        public let podcasts: [Podcast]

        private enum CodingKeys: String, CodingKey {
            case title
            case country
            case _updated = "updated"
            case podcasts = "results"
        }
    }

    public let feed: Feed
}

public typealias Feed = RSSFeed.Feed
public typealias Podcast = Feed.Podcast

let json = """
{
"feed": {
"title":"Top Audio Podcasts",
"country":"gb",
"updated":"2017-11-16T02:02:55.000-08:00",
"results":[
{
"artistName":"BBC Radio",
"name":"Blue Planet II: The Podcast",
"releaseDate":"2017-11-12",
"url":"https://itunes.apple.com/gb/podcast/blue-planet-ii-the-podcast/id1296222557?mt=2"
},
{
"artistName":"Audible",
"name":"The Butterfly Effect with Jon Ronson",
"releaseDate":"2017-11-03",
"url":"https://itunes.apple.com/gb/podcast/the-butterfly-effect-with-jon-ronson/id1258779354?mt=2"
},
{
"artistName":"TED",
"name":"TED Talks Daily",
"releaseDate":"2017-11-16",
"url":"https://itunes.apple.com/gb/podcast/ted-talks-daily/id160904630?mt=2"
}
]
}
}
"""

let data = Data(json.utf8)
let decoder = JSONDecoder()
let rssFeed = try! decoder.decode(RSSFeed.self, from: data)

let feed = rssFeed.feed
print(feed.title, feed.country, feed.updated)

feed.podcasts.forEach {
    print($0.name)
}