Assuming you have a date field which allow users to choose a date for an event. You want to display date in a specific format which matches your country date format (for example 29 September, 2015) and store the value in the database in another format (for example 2015-09-29) to allow correct ordering and doing queries with dates.

In this case, you can do that using the rwmb_{$field_id}_value and rwmb_{$field_id}_meta filters (see this documentation) to change the displayed value and saved value. Here is the sample code:

add_filter( 'rwmb_ID_value', function( $value ) {
    return $value ? date( 'Y-m-d', strtotime( $value ) ) : '';
} );
add_filter( 'rwmb_ID_field_meta', function( $value ) {
    return $value ? date( 'd F, Y', strtotime( $value ) ) : '';
} );

Don’t forget to change ID to your field’s ID.

Also note that the code above change the date format in the backend only. To show the date in the frontend in a different format, please use the code below:

$meta = rwmb_meta( 'field_id' );
echo date( 'd F, Y', strtotime( $meta ) );