custom calendar with jetpack compose & java.time

Calendar.kt

private val DaysInMonthCell = GridCells.Fixed(count = 7)

@Composable
fun Calendar(
    modifier: Modifier = Modifier,
    localDateTime: LocalDateTime
) {
    val firstDate = remember(localDateTime) {
        with(localDateTime) {
            val firstOfMonth = withDayOfMonth(1)
            val firstDayOfFirstWeek = firstOfMonth.dayOfWeek.value
            firstOfMonth.minusDays(firstDayOfFirstWeek.toLong())
        }
    }

    val daysOfWeek = remember {
        DateFormatSymbols().shortWeekdays.drop(1)
    }

    Column(modifier = modifier.fillMaxSize()) {
        Row(modifier = Modifier.fillMaxWidth()) {
            daysOfWeek.forEach { dayOfWeek ->
                key(dayOfWeek) {
                    Text(
                        text = dayOfWeek,
                        modifier = Modifier.weight(1F),
                        textAlign = TextAlign.Center,
                        maxLines = 1
                    )
                }
            }
        }
        LazyVerticalGrid(columns = DaysInMonthCell) {
            items(42) { index ->
                val date = firstDate.plusDays(index.toLong())
                val isInCurrentMonth = date.month == localDateTime.month
                val isToday =
                    date.dayOfYear == LocalDateTime.now().dayOfYear && date.year == LocalDateTime.now().year

                Text(
                    text = date.dayOfMonth.toString(),
                    textAlign = TextAlign.Center,
                    color = if (isToday)
                        Color.Red
                    else
                        if (isInCurrentMonth)
                            MaterialTheme.colors.primary
                        else
                            MaterialTheme.colors.secondary
                )
            }
        }
    }
}

MainActivity.kt

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            MyHabitTheme {
                Icons.Outlined.Create
                // A surface container using the 'background' color from the theme
                Surface(
                    modifier = Modifier.fillMaxSize(),
                    color = MaterialTheme.colors.background
                ) {
                    Column {
                        var localDateTime by remember { mutableStateOf(LocalDateTime.now()) }

                        Row(
                            horizontalArrangement = Arrangement.SpaceEvenly,
                            modifier = Modifier.fillMaxWidth(),
                            verticalAlignment = Alignment.CenterVertically
                        ) {
                            Button(onClick = { localDateTime = localDateTime.minusMonths(1) }) {
                                Text(text = "Prev")
                            }
                            Text(text = "${localDateTime.month.name} ${localDateTime.year}")
                            Button(onClick = { localDateTime = localDateTime.plusMonths(1) }) {
                                Text(text = "Next")
                            }
                        }
                        Calendar(localDateTime = localDateTime)
                    }
                }
            }
        }
    }
}