subtract two timePickerDialog

Question

I have two buttons that open two timePickerDialogs, one is the start time and the other is the end time. How can I extract the difference in hours and minutes between timePickerDialogIni and timePickerDialogFin?

in declarations

var mHourIni: Int = 0
var mMinuteIni: Int = 0
var mHourFin: Int = 0
var mMinuteFin: Int = 0

in metod onCkick

R.id.btn_timeIni ->{
            mHourIni = calendar.get(Calendar.HOUR)
            mMinuteIni = calendar.get(Calendar.MINUTE)
            val timePickerDialogIni = TimePickerDialog(this,
                { view, hourOfDay, minute -> btnTimeIni!!.setText( hourOfDay.toString() + ":" + String.format("%02d", minute))},
                mHourIni!!, mMinuteIni!!, true)//

            timePickerDialogIni.show()
        }

        R.id.btn_timeFin ->{

            mHourFin = calendar.get(Calendar.HOUR)
            mMinuteFin = calendar.get(Calendar.MINUTE)

            val timePickerDialogFin = TimePickerDialog(this,
                { view, hourOfDayFin, minuteFin -> btnTimeFin!!.setText ( hourOfDayFin.toString() + String.format("%02d", minuteFin) +"" )},
                mHourFin!!, mMinuteFin!!, true)

            timePickerDialogFin.show()
        }