Wordpress/PHP获取两个日期之间的天数并创建数组

custom-post-type

function get_vacation_dates(){
global $post;

  $args = array(
    'post_type' => array('vacationperiods'),
    'post_status' => array('publish'),
    'order' => 'ASC'
  );

  $posts = get_posts($args);
  $vac = [];
  foreach ($posts as $post){

    $vacationTitle = get_field('name', $post->ID);
    $startDate = get_field('vacation_start', $post->ID);
    $endDate = get_field('vacation_end', $post->ID);

    // GET ALL DAYS/DATES
    for ($i=$startDate; $i<=$endDate; $i+=86400) {  
        $days = date('Y-m-d', $i);
    }       

    $duration = dateDiff($startDate, $endDate);

    $vac[] = [
        'name' => $vacationTitle,
        'start' => $startDate,
        'end' => $endDate,
        'duration' => $duration,
        'days' => $days
    ];



  }

  echo json_encode($days);

  die();
}

这种方法不起作用,它给我 "days: '1970-01-01'" ['2018-09-13', '2018-09-14', etc..]

我怎样才能做到这一点?