DateFormatter.date(from:) doesn't work in TimelineProvider.getTimeline(in:completion:)

Code Block language
        let formatter = DateFormatter()
        formatter.locale = Locale(identifier: "en_US_POSIX")
        formatter.dateFormat = "yyyyMMdd"
        let date = formatter.date(from: "20201006")
        let string = formatter.string(from: Date())

Code Block 
date = (Date?) nil
string = (String) "20201006"