add_action('save_post', 'change_content');
global $post;
global $wpdb;
function change_content($post_id) {
$datefield = get_post_meta($post_id,'something_different',true);
$post_date = date("Y-m-d H:i:s", strtotime($datefield));
$my_post = array();
$my_post['ID'] = $post_id;
$my_post['post_date'] = $post_date;

remove_action('save_post', 'change_content');
    wp_update_post( $my_post );
add_action('save_post', 'change_content');

https://support.advancedcustomfields.com/forums/topic/set-post-publish-date-by-custom-field/

I’m using the code above,but get_post_meta doesn’t work.
How can I get it to work? By the way I don’t use ACF.