Set default value of field to another field already entered

Hello,

This can be done with custom JavaScript. Here is the pseudo-code:

First step: enqueue your JS file:

add_action( 'rwmb_enqueue_scripts', function() {
    wp_enqueue_script( 'your-js-id', 'url-to-your-js', ['jquery'], '', true );
} );

And in your JS file:

jQuery(function(){
    var $start = $( '#start_date' ),
        $end = $( '#end_date' );
    $start.datepicker( {
        onSelect: function( dateText ) {
            $end.val( dateText );
        }
    } );
});