Converts a formatted Ninja Forms date to a timestamp for compatibility with Custom Fields (Date Type)

nf-create-post-wp-types-date.php

<?php

// Requires PHP v5.3 or higher.
add_filter( 'ninja_forms_create_post_meta_value', function( $meta_value, $post_type, $meta_key ){
  
    // TODO: Update $post_type check.
    // TODO: Update $meta_key  check.
    if( 'test' == $post_type && 'wpcf-test-date' == $meta_key ){
      
        // Convert formatted date to timestamp for WP-Types compatibility.
        $meta_value = DateTime::createFromFormat( 'd/m/Y', $meta_value )->getTimestamp();
    }
    return $meta_value;
}, 10, 3 );