You can use two different date formatters (with and without fraction seconds) and create a custom DateDecodingStrategy. In case of failure when parsing the date returned by the API you can throw a DecodingError as suggested by @PauloMattos in comments:

iOS 9, macOS 10.9, tvOS 9, watchOS 2, Xcode 9 or later

The custom ISO8601 DateFormatter:

extension Formatter {    static let iso8601withFractionalSeconds: DateFormatter = {        let formatter = DateFormatter()        formatter.calendar = Calendar(identifier: .iso8601)        formatter.locale = Locale(identifier: "en_US_POSIX")        formatter.timeZone = TimeZone(secondsFromGMT: 0)        formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSXXXXX"        return formatter    }()    static let iso8601: DateFormatter = {        let formatter = DateFormatter()        formatter.calendar = Calendar(identifier: .iso8601)        formatter.locale = Locale(identifier: "en_US_POSIX")        formatter.timeZone = TimeZone(secondsFromGMT: 0)        formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ssXXXXX"        return formatter    }()}

The custom DateDecodingStrategy:

extension JSONDecoder.DateDecodingStrategy {    static let customISO8601 = custom {        let container = try $0.singleValueContainer()        let string = try container.decode(String.self)        if let date = Formatter.iso8601withFractionalSeconds.date(from: string) ?? Formatter.iso8601.date(from: string) {            return date        }        throw DecodingError.dataCorruptedError(in: container, debugDescription: "Invalid date: \(string)")    }}

The custom DateEncodingStrategy:

extension JSONEncoder.DateEncodingStrategy {    static let customISO8601 = custom {        var container = $1.singleValueContainer()        try container.encode(Formatter.iso8601withFractionalSeconds.string(from: $0))    }}

edit/update:

Xcode 10 • Swift 4.2 or later • iOS 11.2.1 or later

ISO8601DateFormatter now supports formatOptions .withFractionalSeconds:

extension Formatter {    static let iso8601withFractionalSeconds: ISO8601DateFormatter = {        let formatter = ISO8601DateFormatter()        formatter.formatOptions = [.withInternetDateTime, .withFractionalSeconds]        return formatter    }()    static let iso8601: ISO8601DateFormatter = {        let formatter = ISO8601DateFormatter()        formatter.formatOptions = [.withInternetDateTime]        return formatter    }()}

The customs DateDecodingStrategy and DateEncodingStrategy would be the same as shown above.

// Playground testingstruct ISODates: Codable {    let dateWith9FS: Date    let dateWith3FS: Date    let dateWith2FS: Date    let dateWithoutFS: Date}

let isoDatesJSON = """{"dateWith9FS": "2017-06-19T18:43:19.532123456Z","dateWith3FS": "2017-06-19T18:43:19.532Z","dateWith2FS": "2017-06-19T18:43:19.53Z","dateWithoutFS": "2017-06-19T18:43:19Z",}"""

let isoDatesData = Data(isoDatesJSON.utf8)let decoder = JSONDecoder()decoder.dateDecodingStrategy = .customISO8601do {    let isoDates = try decoder.decode(ISODates.self, from: isoDatesData)    print(Formatter.iso8601withFractionalSeconds.string(from: isoDates.dateWith9FS))   // 2017-06-19T18:43:19.532Z    print(Formatter.iso8601withFractionalSeconds.string(from: isoDates.dateWith3FS))   // 2017-06-19T18:43:19.532Z    print(Formatter.iso8601withFractionalSeconds.string(from: isoDates.dateWith2FS))   // 2017-06-19T18:43:19.530Z    print(Formatter.iso8601withFractionalSeconds.string(from: isoDates.dateWithoutFS)) // 2017-06-19T18:43:19.000Z} catch {    print(error)}

Swift 5

To parse ISO8601 string to date you have to use DateFormatter. In newer systems (f.ex. iOS11+) you can use ISO8601DateFormatter.

As long as you don't know if date contains milliseconds, you should create 2 formatters for each case. Then, during parsing String to Date use both consequently.

DateFormatter for older systems

/// Formatter for ISO8601 with millisecondslazy var iso8601FormatterWithMilliseconds: DateFormatter = {    let dateFormatter = DateFormatter()    dateFormatter.locale = Locale(identifier: "en_US_POSIX")    dateFormatter.timeZone = TimeZone(abbreviation: "GMT")    dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSZ"    return dateFormatter}()/// Formatter for ISO8601 without millisecondslazy var iso8601Formatter: DateFormatter = {    let dateFormatter = DateFormatter()    dateFormatter.locale = Locale(identifier: "en_US_POSIX")    dateFormatter.timeZone = TimeZone(abbreviation: "GMT")    dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ssZZZZZ"    return dateFormatter}()

ISO8601DateFormatter for newer systems (f.ex. iOS 11+)

lazy var iso8601FormatterWithMilliseconds: ISO8601DateFormatter = {    let formatter = ISO8601DateFormatter()    // GMT or UTC -> UTC is standard, GMT is TimeZone    formatter.timeZone = TimeZone(abbreviation: "GMT")    formatter.formatOptions = [.withInternetDateTime,                               .withDashSeparatorInDate,                               .withColonSeparatorInTime,                               .withTimeZone,                               .withFractionalSeconds]    return formatter}()/// Formatter for ISO8601 without millisecondslazy var iso8601Formatter: ISO8601DateFormatter = {    let formatter = ISO8601DateFormatter()    // GMT or UTC -> UTC is standard, GMT is TimeZone    formatter.timeZone = TimeZone(abbreviation: "GMT")    formatter.formatOptions = [.withInternetDateTime,                               .withDashSeparatorInDate,                               .withColonSeparatorInTime,                               .withTimeZone]    return formatter}()

Summary

As you can notice there is 2 formatters to create. If you want to support older systems, it makes 4 formatters. To make it more simple, check out Tomorrow on GitHub where you can see entire solution for this problem.

To convert String to Date you use:

let date = Date.fromISO("2020-11-01T21:10:56.22+02:00")

A new option (as of Swift 5.1) is a Property Wrapper. The CodableWrappers library has an easy way to deal with this.

For default ISO8601

@ISO8601DateCoding struct JustADate: Codable {    var date: Date }

If you want a custom version:

// Custom coder@available(macOS 10.12, iOS 10.0, watchOS 3.0, tvOS 10.0, *)public struct FractionalSecondsISO8601DateStaticCoder: StaticCoder {    private static let iso8601Formatter: ISO8601DateFormatter = {        let formatter = ISO8601DateFormatter()        formatter.formatOptions = .withFractionalSeconds        return formatter    }()    public static func decode(from decoder: Decoder) throws -> Date {        let stringValue = try String(from: decoder)        guard let date = iso8601Formatter.date(from: stringValue) else {            throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Expected date string to be ISO8601-formatted."))        }        return date    }    public static func encode(value: Date, to encoder: Encoder) throws {        try iso8601Formatter.string(from: value).encode(to: encoder)    }}// Property Wrapper aliaspublic typealias ISO8601FractionalDateCoding = CodingUses<FractionalSecondsISO8601DateStaticCoder>// Usage@ISO8601FractionalDateCodingstruct JustADate: Codable {    var date: Date }