Android MaterialDatePicker, MaterialTimePicker tutorial - https://kennay-kermani.medium.com/android-material-date-picker-and-material-time-picker-in-practice-11c01582e52e

activity_main.xml

<?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/output"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="--"
        android:textSize="20sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <com.google.android.material.button.MaterialButton
        android:id="@+id/select_button"
        android:layout_width="wrap_content"
        android:layout_height="0dp"
        android:text="Select"
        android:layout_marginTop="16dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/output" />
</androidx.constraintlayout.widget.ConstraintLayout>

build.gradle

implementation 'com.google.android.material:material:1.3.0'

MaterialDatePickerMultipleSelection-MainActivity.java

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.TextView
import androidx.core.util.Pair
import com.google.android.material.button.MaterialButton
import com.google.android.material.datepicker.MaterialDatePicker
import java.time.Instant
import java.time.LocalDateTime
import java.time.ZoneId
import java.time.format.DateTimeFormatter

class MainActivity : AppCompatActivity() {

    private var currentSelectedStartDate: Long? = null
    private var currentSelectedEndDate: Long? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        setClickListeners()
    }

    private fun setClickListeners() {
        findViewById<MaterialButton>(R.id.select_button).setOnClickListener { showDatePicker() }
    }

    private fun showDatePicker() {
        val selectedStartDateInMillis = currentSelectedStartDate ?: System.currentTimeMillis()
        val selectedEndDateInMillis = currentSelectedEndDate ?: System.currentTimeMillis()
        val dateRange = androidx.core.util.Pair(selectedStartDateInMillis, selectedEndDateInMillis)
        
        MaterialDatePicker.Builder.dateRangePicker().setSelection(dateRange).build().apply {
            addOnPositiveButtonClickListener { selectedDateRangePair -> onDateRangeSelected(selectedDateRangePair) }
        }.show(supportFragmentManager, MaterialDatePicker::class.java.canonicalName)
    }

    private fun onDateRangeSelected(dateTimeStampInMillis: Pair<Long, Long>) {
        currentSelectedStartDate = dateTimeStampInMillis.first ?: System.currentTimeMillis()
        currentSelectedEndDate = dateTimeStampInMillis.second ?: System.currentTimeMillis()
      
        val startDateTime: LocalDateTime = LocalDateTime.ofInstant(Instant.ofEpochMilli(currentSelectedStartDate), ZoneId.systemDefault())
        val endDateTime: LocalDateTime = LocalDateTime.ofInstant(Instant.ofEpochMilli(currentSelectedEndDate), ZoneId.systemDefault())
        
        val datePattern = "yyyy-MM-dd"
        val startDateFormatted: String = startDateTime.format(DateTimeFormatter.ofPattern(datePattern))
        val endDateFormatted: String = endDateTime.format(DateTimeFormatter.ofPattern(datePattern))

        "$startDateFormatted -- $endDateFormatted".also { findViewById<TextView>(R.id.output).text = it }
    }
}

MaterialDatePickerSingleSelection-MainActivity.java

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.TextView
import com.google.android.material.button.MaterialButton
import com.google.android.material.datepicker.MaterialDatePicker
import java.time.Instant
import java.time.LocalDateTime
import java.time.ZoneId
import java.time.format.DateTimeFormatter

class MainActivity : AppCompatActivity() {

    private var currentSelectedDate: Long? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        setClickListeners()
    }

    private fun setClickListeners() {
        findViewById<MaterialButton>(R.id.select_button).setOnClickListener { showDatePicker() }
    }

    private fun showDatePicker() {
        val selectedDateInMillis = currentSelectedDate ?: System.currentTimeMillis()
        
        MaterialDatePicker.Builder.datePicker().setSelection(selectedDateInMillis).build().apply {
            addOnPositiveButtonClickListener { dateInMillis -> onDateSelected(dateInMillis) }
        }.show(supportFragmentManager, MaterialDatePicker::class.java.canonicalName)
    }

    private fun onDateSelected(dateTimeStampInMillis: Long) {
        currentSelectedDate = dateTimeStampInMillis
        val dateTime: LocalDateTime = LocalDateTime.ofInstant(Instant.ofEpochMilli(currentSelectedDate), ZoneId.systemDefault())
        val dateAsFormattedText: String = dateTime.format(DateTimeFormatter.ofPattern("yyyy-MM-dd"))
        findViewById<TextView>(R.id.output).text = dateAsFormattedText
    }
}

system-time-format

val usesSystem24HFormat = is24HourFormat(this)
val timeFormat = if (usesSystem24HFormat) TimeFormat.CLOCK_24H else TimeFormat.CLOCK_12H

TimeInput-MainActivity.java

.setInputMode(INPUT_MODE_KEYBOARD)

TimePicker-MainActivity.java

import android.os.Bundle
import android.widget.TextView
import androidx.appcompat.app.AppCompatActivity
import com.google.android.material.button.MaterialButton
import com.google.android.material.timepicker.MaterialTimePicker
import com.google.android.material.timepicker.TimeFormat
import java.time.LocalDateTime

class MainActivity : AppCompatActivity() {

    private var selectedHour: Int? = null
    private var selectedMinute: Int? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        setClickListeners()
    }

    private fun setClickListeners() {
        findViewById<MaterialButton>(R.id.select_button).setOnClickListener { showTimePicker() }
    }

    private fun showTimePicker() {
        val hour = selectedHour ?: LocalDateTime.now().hour
        val minute = selectedMinute ?: LocalDateTime.now().minute

        MaterialTimePicker.Builder()
            .setTimeFormat(TimeFormat.CLOCK_24H)
            .setHour(hour)
            .setMinute(minute)
            .build()
            .apply {
                addOnPositiveButtonClickListener { onTimeSelected(this.hour, this.minute) }
            }.show(supportFragmentManager, MaterialTimePicker::class.java.canonicalName)
    }

    private fun onTimeSelected(hour: Int, minute: Int) {
        selectedHour = hour
        selectedMinute = minute
        val hourAsText = if (hour < 10) "0$hour" else hour
        val minuteAsText = if (minute < 10) "0$minute" else minute

        "$hourAsText:$minuteAsText".also { findViewById<TextView>(R.id.output).text = it }
    }
}