Hi! This done the job for me :
Paste this function on your functions.php theme.
// Save ACF custom field to date-time post
function my_acf_save_post( $post_id ) {
$acfDate = get_field('initial_time_var', $post_id);
//Test if you receive the data field correctly:
//echo $acfDate;
//exit (-1);
$my_post = array();
$my_post['ID'] = $post_id;
$my_post['post_date'] = $acfDate;
wp_update_post( $my_post );
}
add_action('acf/save_post', 'my_acf_save_post', 20);
Notes: Be sure to give the correct post id variable ($post_id) and the correct time variable (initial_time_var for me)
Important: Set the field type (data picker) in your ACF panel to return this custom exact format: Y-m-d H:i:s