How to change string to date without converting to any timezone in Swift?

extension Date {
    func originToString(dateFormat: String) -> String {
        let dateFormatter = DateFormatter()
        dateFormatter.dateFormat = dateFormat
        dateFormatter.calendar = Calendar(identifier: .iso8601)
        dateFormatter.locale = Locale(identifier: "en_US_POSIX")
        dateFormatter.timeZone = TimeZone(secondsFromGMT: 0)
        return dateFormatter.string(from: self)
    }
}

let dateString = "19-08-2015 09:00 AM"
let dateFormatter = DateFormatter()
dateFormatter.calendar = Calendar(identifier: .iso8601)
dateFormatter.locale = Locale(identifier: "en_US_POSIX")
dateFormatter.timeZone = TimeZone(secondsFromGMT: 0)
dateFormatter.dateFormat = "dd-MM-yyyy hh:mm a"
if let dateFromString = dateFormatter.date(from: dateString) {
    print(dateFromString)   // "2015-08-19 09:00:00 +0000"
    dateFormatter.timeZone = .current
    dateFormatter.dateFormat = "dd-MM-yyyy hh:mm a Z"
    dateFormatter.string(from: dateFromString)  // 19-08-2015 06:00 AM -0300"
}