Attachment: Download

Issue Description:

The following piece of code succeeds on Darwin, but fails on Linux:

import Foundation
import Dispatch

let currentDate = Date()
let formatter = ISO8601DateFormatter()
let dateStringLength = formatter.string(from: currentDate).count

DispatchQueue.concurrentPerform(iterations: 1000) { _ in
    let formatted = formatter.string(from: currentDate)
    if formatted.count != dateStringLength {
        print("Detected wrong date formatting: \(formatted)")
    }
}

On Linux, using swift-4.1-release it gives:

root@de03da451b4f:/Users/gmilos/Repos/DateFormatterThreadSafety# swift run
Compile Swift Module 'DateFormatterThreadSafety' (1 sources)
Linking ./.build/x86_64-unknown-linux/debug/DateFormatterThreadSafety
Detected wrong date formatting: 2018-0005-23T10:35:12Z
....
Detected wrong date formatting: 2018-05-0023T10:35:12Z
Detected wrong date formatting: 2018-05-23T10:35:0012Z
Detected wrong date formatting: 2018-05-23T0010:35:12Z

I'm attaching reproduction (simple SwiftPM project).

Note that the documentation specifies that date formatter has been thread safe for a while:

https://developer.apple.com/documentation/foundation/dateformatter

Thread Safety

On iOS 7 and later NSDateFormatter is thread safe.

In macOS 10.9 and later NSDateFormatter is thread safe so long as you are using the modern behavior in a 64-bit app.

While it doesn't explicitly talk about Linux (of course), this is the source of truth for most people.