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:

\"2015-01-01T00:0         


        

Answer 1

To further compliment Andrés Torres Marroquín and Leo Dabus, I have a version that preserves fractional seconds. I can't find it documented anywhere, but Apple truncate fractional seconds to the microsecond (3 digits of precision) on both input and output (even though specified using SSSSSSS, contrary to Unicode tr35-31).

I should stress that this is probably not necessary for most use cases. Dates online do not typically need millisecond precision, and when they do, it is often better to use a different data format. But sometimes one must interoperate with a pre-existing system in a particular way.

Xcode 8/9 and Swift 3.0-3.2

extension Date {
    struct Formatter {
        static let iso8601: DateFormatter = {
            let formatter = DateFormatter()
            formatter.calendar = Calendar(identifier: .iso8601)
            formatter.locale = Locale(identifier: "en_US_POSIX")
            formatter.timeZone = TimeZone(identifier: "UTC")
            formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSSSSXXXXX"
            return formatter
        }()
    }

    var iso8601: String {
        // create base Date format 
         var formatted = DateFormatter.iso8601.string(from: self)

        // Apple returns millisecond precision. find the range of the decimal portion
         if let fractionStart = formatted.range(of: "."),
             let fractionEnd = formatted.index(fractionStart.lowerBound, offsetBy: 7, limitedBy: formatted.endIndex) {
             let fractionRange = fractionStart.lowerBound..