OneSignal - 使用变量时发送预定的通知问题

Question

I want to send scheduled messages from OneSignal, code works fine if I use send_after with hard code time. But when I replace time with my time variable , it's not working. I didn't get the exact problem . Please help.

Thanks in advance.

    $fields = array(
  'app_id' => "XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX",
  'included_segments' => array('All'),
  'data' => array("foo" => "bar"),
  'contents' => $content,
  'send_after' => "Sept 24 2017 14:00:00 GMT-0700"
);

above code works but below code doesn't

    $fields = array(
  'app_id' => "XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX",
  'included_segments' => array('All'),
  'data' => array("foo" => "bar"),
  'contents' => $content,
  'send_after' => "$dateNTime"
);