디데이어플에서 날짜계산이 안됩니다..

실행을 했는데 앱 꺼짐없이 실행은 잘되나

마지막 강의까지 듣고 코드를 수정했는데

갑자기 날짜 계산값이 다 0으로 출력됩니다.

어딘가 오류가 있는 듯 한데.. 못 찾겠네요. ㅠㅠ

package com.sunny.dday

import android.app.DatePickerDialog
import android.os.Build
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.util.Log
import android.widget.Button
import android.widget.DatePicker
import android.widget.TextClock
import android.widget.TextView
import androidx.annotation.RequiresApi
import java.time.Month
import java.util.Calendar
import java.util.GregorianCalendar
import java.util.concurrent.TimeUnit

class MainActivity : AppCompatActivity() {

    @RequiresApi(Build.VERSION_CODES.N)
    override fun onCreate(savedInstanceState: Bundle?) {

        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val startButton = findViewById<Button>(R.id.startBtn)
        val endButton = findViewById<Button>(R.id.endBtn)
        var startDate = ""
        var endDate = ""

        val calendar_start = Calendar.getInstance()
        val calendar_end = Calendar.getInstance()

        startButton.setOnClickListener {

            val today = GregorianCalendar()
            val year = today.get(Calendar.YEAR)
            val month = today.get(Calendar.MONTH)
            val day = today.get(Calendar.DATE)

            val dlg = DatePickerDialog(this, object : DatePickerDialog.OnDateSetListener {
                override fun onDateSet(view: DatePicker?, year: Int, month: Int, dayOfMonth: Int) {

//                    startDate = "${year} + ${month+1} + ${dayofMonth}"
                    startDate = year.toString() + (month + 1).toString() + dayOfMonth.toString()

                    calendar_start.set(year, month + 1, dayOfMonth)

                    Log.d("day : ", startDate)

                }

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

        endButton.setOnClickListener {
            val today = GregorianCalendar()
            val year = today.get(Calendar.YEAR)
            val month = today.get(Calendar.MONTH)
            val day = today.get(Calendar.DATE)

            val dlg = DatePickerDialog(this, object : DatePickerDialog.OnDateSetListener {
                override fun onDateSet(view: DatePicker?, year: Int, month: Int, dayOfMonth: Int) {

//                    endDate = "${year} + ${month+1} + ${dayofMonth}"
                    endDate = year.toString() + (month + 1).toString() + dayOfMonth.toString()

                    calendar_end.set(year, month + 1, dayOfMonth)

                    val finalDate = TimeUnit.MICROSECONDS.toDays(calendar_end.timeInMillis - calendar_start.timeInMillis)

                    Log.d("day : ", endDate)

                    val textArea = findViewById<TextView>(R.id.finalDate)
                    textArea.setText(finalDate.toString())

                    Log.d("finalday : ", finalDate.toString())
                }
            }, year, month, day)

            dlg.show()
        }
    }
}

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/textView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center"
        android:text="D-Day Counter"
        android:textSize="20dp"
        android:textStyle="bold"
        app:layout_constraintBottom_toTopOf="@+id/imageView"
        app:layout_constraintTop_toTopOf="parent" />

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_marginTop="50dp"
        android:scaleType="fitXY"
        android:src="@drawable/bg"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/finalDate"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center"
        android:text="0 day"
        android:textSize="30sp"
        android:textStyle="bold"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintTop_toTopOf="@+id/textView"
        app:layout_constraintVertical_bias="0.375"
        tools:layout_editor_absoluteX="0dp" />

    <Button
        android:id="@+id/startBtn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_margin="5dp"
        android:background="@drawable/radius"
        android:text="Start Date"
        android:textSize="15sp"
        android:textStyle="bold"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent" />

    <Button
        android:id="@+id/endBtn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_margin="5dp"
        android:layout_marginBottom="4dp"
        android:background="@drawable/radius"
        android:text="D-day date"
        android:textStyle="bold"
        android:textSize="15sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

//                    val finalDate = TimeUnit.MICROSECONDS.toDays(calendar_end.timeInMillis - calendar_start.timeInMillis)
                    val finalDate = TimeUnit.MILLISECONDS.toDays(calendar_end.timeInMillis - calendar_start.timeInMillis)