Date+ISOString.swift

extension Date {

    private static let isoDateFormatter: 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.SSS"

        return dateFormatter
    }()

    func toISOString() -> String {
        Date.isoDateFormatter.string(from: self).appending("Z")
    }
}