I don't understand why you're doing anything with strings...
extension Date {
var millisecondsSince1970:Int64 {
return Int64((self.timeIntervalSince1970 * 1000.0).rounded())
}
init(milliseconds:Int64) {
self = Date(timeIntervalSince1970: TimeInterval(milliseconds) / 1000)
}
}
Date().millisecondsSince1970 // 1476889390939
Date(milliseconds: 0) // "Dec 31, 1969, 4:00 PM" (PDT variant of 1970 UTC)