Q:

How convert timestamp in Kotlin

l am try to convert timeestamp coming from data json url
TimeFlight.text = list[position].TimeFlight.getDateTime(toString())

l am use list view in my app 
override fun getView(position: Int, convertView: View?,  parent: ViewGroup?):  View {

    val view : View = LayoutInflater.from(context).inflate(R.layout.row_layout,parent,false)

    val TimeFlight = view.findViewById(R.id.time_id) as AppCompatTextView
    val LogoAriline = view.findViewById(R.id.logo_image) as ImageView

    status.text= list[position].Stauts
    TimeFlight.text = list[position].TimeFlight.getDateTime(toString())
    Picasso.get().load(Uri.parse("https://www.xxxxxxxxx.com/static/images/data/operators/"+status.text.toString()+"_logo0.png"))
        .into(LogoAriline)

    return view as View
}

private fun getDateTime(s: String): String? { 
    try {
        val sdf = SimpleDateFormat("MM/dd/yyyy")
        val netDate = Date(Long.parseLong(s))
        return sdf.format(netDate)
    } catch (e: Exception) {
        return e.toString()
    }
}

Data class for json 
data class FlightShdu (val Airline : String ,val TimeFlight : String)

l used that code getDateTime but the format unknown

A:

Assuming TimeFlight is a stringified epoch timestamp (in milliseconds), you should pass that to your getDateTime() function:
TimeFlight.text = getDateTime(list[position].TimeFlight)

(if they are not millis but seconds, then simply multiply them by 1000 before passing them to the Date constructor)
On a side note, depending on the exact use case, creating a new SimpleDateFormat object might not be necessary on every getDateTime() call, you can make it an instance variable. 
Also, i'd advise you to take a look at (and follow) the Java naming conventions for both Java and Kotlin applications.