We can make views more dynamic in Android by displaying real-time data in places where we can. This post will illustrate how to display today's date within a calendar icon in the bottom navigation view in Android.

1. Create an icon calendar_today_24, in res\drawable. For this right-click drawable, select New > Vector Asset. Asset type is Clip art, and for Clip art, select Calendar Today icon. This will give the calendar outline icon which has no content inside it.

2. Create a menu, bottom_menu.xml under res\menu with the following content. The first menu item is our calendar where we will display today's date, the day part.

<menu xmlns:android="http://schemas.android.com/apk/res/android">
    <item
        android:id="@+id/nav_today"
        android:icon="@drawable/calendar_today_24"
        android:title="Today" />
    ...

3. Update the MainActivity like below.

public class MainActivity extends AppCompatActivity {
    BottomNavigationView bottomNavigationView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // ..
        bottomNavigationView = findViewById(R.id.bottomNavigationView);
        // ..
        drawTodayIcon();
    }

    private void drawTodayIcon() {
        int dayOfMonth = Calendar.getInstance().get(Calendar.DAY_OF_MONTH);
        // Generate an icon with the day inside the calendar
        Drawable calendarIcon = ContextCompat.getDrawable(this, R.drawable.calendar_today_24);
        if (calendarIcon != null) {
            Bitmap dayIcon = genCalendarIcon(calendarIcon, String.valueOf(dayOfMonth));
            // Set the generated icon to the menu item
            bottomNavigationView.getMenu().findItem(R.id.nav_today)
                    .setIcon(new BitmapDrawable(getResources(), dayIcon));
        }
    }

    private Bitmap genCalendarIcon(Drawable baseIcon, String dayText) {
        int width = baseIcon.getIntrinsicWidth();
        int height = baseIcon.getIntrinsicHeight();

        // Ensure valid size
        if (width <= 0 || height <= 0) {
            width = 100;
            height = 100;
        }

        Bitmap bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
        Canvas canvas = new Canvas(bitmap);

        // Draw the base calendar icon
        baseIcon.setBounds(0, 0, width, height);
        baseIcon.draw(canvas);

        // Prepare paint for the day number
        Paint paint = new Paint();
        paint.setTextSize(height / 2.5f);
        paint.setTypeface(Typeface.DEFAULT_BOLD);
        paint.setAntiAlias(true);
        paint.setTextAlign(Paint.Align.CENTER);

        // Calculate position to center the text
        float xPos = width / 2f;
        float yPos = (height / 2f) + (height / 4f);

        // Draw the day number on top of the calendar icon
        canvas.drawText(dayText, xPos, yPos, paint);

        return bitmap;
    }
}

With this we will have a nice calendar icon with date inside it which changes dynamically in the bottom navigation menu view.