How can I parse / create a date time stamp formatted with fractional seconds UTC timezone (ISO 8601, RFC 3339) in Swift?

Question

How to generate a date time stamp, using the format standards for ISO 8601 and RFC 3339?

The goal is a string that looks like this:

"2015-01-01T00:00:00.000Z"

Format:

• year, month, day, as "XXXX-XX-XX"
• the letter "T" as a separator
• hour, minute, seconds, milliseconds, as "XX:XX:XX.XXX".
• the letter "Z" as a zone designator for zero offset, a.k.a. UTC, GMT, Zulu time.

Best case:

• Swift source code that is simple, short, and straightforward.
• No need to use any additional framework, subproject, cocoapod, C code, etc.

I've searched StackOverflow, Google, Apple, etc. and haven't found a Swift answer to this.

The classes that seem most promising are NSDate, NSDateFormatter, NSTimeZone.

Related Q&A: How do I get an ISO 8601 date on iOS?

Here's the best I've come up with so far:

var now = NSDate()
var formatter = NSDateFormatter()
formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'"
formatter.timeZone = NSTimeZone(forSecondsFromGMT: 0)
println(formatter.stringFromDate(now))

Answer 1

Swift 5.5 • iOS 15 • Xcode 13 or later

extension Date.ISO8601FormatStyle {
    static let iso8601withFractionalSeconds: Self = .init(includingFractionalSeconds: true)
}

---

extension ParseStrategy where Self == Date.ISO8601FormatStyle {
    static var iso8601withFractionalSeconds: Date.ISO8601FormatStyle { .iso8601withFractionalSeconds }
}

---

extension FormatStyle where Self == Date.ISO8601FormatStyle {
    static var iso8601withFractionalSeconds: Date.ISO8601FormatStyle { .iso8601withFractionalSeconds }
}

---

extension Date {

    init(iso8601withFractionalSeconds parseInput: ParseStrategy.ParseInput) throws {
        try self.init(parseInput, strategy: .iso8601withFractionalSeconds)
    }

    var iso8601withFractionalSeconds: String {
        formatted(.iso8601withFractionalSeconds)
    }
}

---

extension String {
    func iso8601withFractionalSeconds() throws -> Date {
        try .init(iso8601withFractionalSeconds: self)
    }
}

---

extension JSONDecoder.DateDecodingStrategy {
    static let iso8601withFractionalSeconds = custom {
        try .init(iso8601withFractionalSeconds: $0.singleValueContainer().decode(String.self))
    }
}

---

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

---

Usage:

let date: Date = .now  // "19 Nov 2023 at 11:29 PM"
date.description(with: .current)  // "Sunday, 19 November 2023 at 11:29:40 PM Brasilia Standard Time"
let dateString = date.iso8601withFractionalSeconds  // "2023-11-20T02:29:40.920Z"

if let date = try? dateString.iso8601withFractionalSeconds() {
    date.description(with: .current) // "Sunday, 19 November 2023 at 11:29:40 PM Brasilia Standard Time"
    print(date.iso8601withFractionalSeconds)  // "2023-11-20T02:29:40.920Z\n"
}

---

---

Swift 4 • iOS 11.2.1 or later

extension ISO8601DateFormatter {
    convenience init(_ formatOptions: Options) {
        self.init()
        self.formatOptions = formatOptions
    }
}

---

extension Formatter {
    static let iso8601withFractionalSeconds = ISO8601DateFormatter([.withInternetDateTime, .withFractionalSeconds])
}

---

extension Date {
    var iso8601withFractionalSeconds: String { return Formatter.iso8601withFractionalSeconds.string(from: self) }
}

---

extension String {
    var iso8601withFractionalSeconds: Date? { return Formatter.iso8601withFractionalSeconds.date(from: self) }
}

---

Usage:

Date().description(with: .current)  //  Tuesday, February 5, 2019 at 10:35:01 PM Brasilia Summer Time"
let dateString = Date().iso8601withFractionalSeconds   //  "2019-02-06T00:35:01.746Z"

if let date = dateString.iso8601withFractionalSeconds {
    date.description(with: .current) // "Tuesday, February 5, 2019 at 10:35:01 PM Brasilia Summer Time"
    print(date.iso8601withFractionalSeconds)           //  "2019-02-06T00:35:01.746Z\n"
}

---

let dates: [Date] = [.now]

let encoder = JSONEncoder()
encoder.dateEncodingStrategy = .iso8601withFractionalSeconds
let data = try! encoder.encode(dates)
print(String(data: data, encoding: .utf8)!)  // "["2023-11-20T02:11:29.158Z"]\n"

let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .iso8601withFractionalSeconds
let decodedDates = try! decoder.decode([Date].self, from: data)
print(decodedDates)   // "[2023-11-20 02:11:29 +0000]\n"

---

iOS 9 • Swift 3 or later

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
    }()
}

---

Codable Protocol

If you need to encode and decode this format when working with Codable protocol you can create your own custom date encoding/decoding strategies:

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

and the encoding strategy

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

---

Playground Testing

let dates = [Date()]   // ["Feb 8, 2019 at 9:48 PM"]

encoding

let encoder = JSONEncoder()
encoder.dateEncodingStrategy = .iso8601withFractionalSeconds
let data = try! encoder.encode(dates)
print(String(data: data, encoding: .utf8)!)

decoding

let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .iso8601withFractionalSeconds
let decodedDates = try! decoder.decode([Date].self, from: data)  // ["Feb 8, 2019 at 9:48 PM"]