인프런 커뮤니티 질문&답변

package com.example.diet_memo

import android.app.DatePickerDialog
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.view.LayoutInflater
import android.widget.Button
import android.widget.DatePicker
import android.widget.ImageView
import androidx.appcompat.app.AlertDialog
import java.util.*

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val writeButton = findViewById<ImageView>(R.id.writebtn)
        writeButton.setOnClickListener{

            val mDialogView = LayoutInflater.from(this).inflate(R.layout.custom_dialog,null)
            val mBuilder = AlertDialog.Builder(this)
                .setView(mDialogView)
                .setTitle("운동 메모 다이얼로그")
            val mAlertDialog = mBuilder.show()
            val Dateselectbtn = mAlertDialog.findViewById<Button>(R.id.selectdate)

            Dateselectbtn?.setOnClickListener{
                val today = GregorianCalendar()
                val year : Int = today.get(Calendar.YEAR)
                val month : Int = today.get(Calendar.MONTH)
                val date : Int = today.get(Calendar.DATE)

                val dlg = DatePickerDialog(this,object: DatePickerDialog.OnDateSetListener{
                    override fun onDateSet(p0: DatePicker?, p1: Int, p2: Int, p3: Int) {

                        Dateselectbtn.setText("${year},${month + 1},${dayOfMonth}")

                    }

                },year,month,date)
                dlg.show()
            }


여기에서 dayOfMonth 부분에 빨간 에러가 뜨는데 

에러가 뜨는 이유가 궁금합니다.

항상 강의 잘 듣고 있습니다. 감사합니다!!