MyActivity.kt

class MyActivity : AppCompatActivity() {

    lateinit var date_button: Button

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.my_activity_layout)

        date_button = findViewById(R.id.my_date_button)
    }

    fun showDatePickerDialog(v: View) {
        val newFragment = DatePickerFragment()
        newFragment.show(supportFragmentManager, "datePicker")
    }

    fun updateDate(updated_date: String) {
        this.date_button.text = updated_date
    }
}