Android 标题栏根据滑动更改标题（月份和年份）

我的导航抽屉标题栏标题显示当前月份和年份，我想在用户每次刷卡时更改标题。假设目前是2015年6月；当用户向左滑动时，显示为2015年5月，当用户向右滑动时，显示为2015年7月。你知道怎么做吗？我得到了当前日期和时间，如下所示：

public class MenuDrawer extends ActionBarActivity implements FragmentDrawer.FragmentDrawerListener {

private static String TAG = MenuDrawer.class.getSimpleName();
private Calendar cal = Calendar.getInstance();
private SimpleDateFormat dateFormatter = new SimpleDateFormat("MMMM yyyy");

private Toolbar mToolbar;
private FragmentDrawer drawerFragment;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_menubar);

    mToolbar = (Toolbar) findViewById(R.id.toolbar);

    setSupportActionBar(mToolbar);
    getSupportActionBar().setDisplayShowHomeEnabled(true);

    drawerFragment = (FragmentDrawer) getSupportFragmentManager().findFragmentById(R.id.fragment_navigation_drawer);
    drawerFragment.setUp(R.id.fragment_navigation_drawer, (DrawerLayout) findViewById(R.id.drawer_layout), mToolbar);
    drawerFragment.setDrawerListener(this);

    // display the first navigation drawer view on app launch
    displayView(0);
}

@Override
public void onDrawerItemSelected(View view, int position) {
    displayView(position);
}

private void displayView(int position) {
    Fragment fragment = null;
    String title = getString(R.string.app_name);

    switch (position) {
        case 0:
            fragment = new CalendarFragment();
            // get current month and year
            title = dateFormatter.format(cal.getTime());
            break;

        default: break;
    }

    if (fragment != null) {
        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.replace(R.id.container_body, fragment);
        fragmentTransaction.commit();

        // set the toolbar title
        getSupportActionBar().setTitle(title);
    }
}

}