ios - Date to milliseconds and back to date in Swift

Question

I am taking the current time, in UTC, and putting it in nanaoseconds and then I need to take the nanoseconds and go back to a date in local time. I am able to do get the time to nanoseconds and then back to a date string but the time gets convoluted when I go from a string to date.

    //Date to milliseconds
     func currentTimeInMiliseconds() -> Int! {
            let currentDate = NSDate()
            let dateFormatter = DateFormatter()
            dateFormatter.dateFormat = format
            dateFormatter.timeZone = NSTimeZone(name: "UTC") as TimeZone!
            let date = dateFormatter.date(from: dateFormatter.string(from: currentDate as Date))
            let nowDouble = date!.timeIntervalSince1970
            return Int(nowDouble*1000)
        }

    //Milliseconds to date
    extension Int {
        func dateFromMilliseconds(format:String) -> Date {
            let date : NSDate! = NSDate(timeIntervalSince1970:Double(self) / 1000.0)
            let dateFormatter = DateFormatter()
            dateFormatter.dateFormat = format
            dateFormatter.timeZone = TimeZone.current
            let timeStamp = dateFormatter.string(from: date as Date)

let formatter = DateFormatter()
            formatter.dateFormat = format
            return ( formatter.date( from: timeStamp ) )!
        }
    }

//The timestamp is correct but the date returned isn't

Answer 1

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)