سلام
تست کنید:
$get_the_modified_date = gregdate('Y-m-d',get_the_modified_date('Y-m-d'),'eng');
$post_date = date_create($get_the_modified_date);
$current_date = date_create(date('Y-m-d'));
if($post_date != false){
$diff = date_diff($post_date,$current_date);
$number_of_day = $diff->format("%R%a days");
}