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function check_if_new( $atts ) {
	$shortcode_atts = (shortcode_atts(array(
      'post_id' => "1",
   ), $atts));
   $reqid = $shortcode_atts[ 'post_id' ];
	$queried_post = get_post($reqid); 
	$post_date = $queried_post->post_date;
	$out = '';
	if ( strtotime( $post_date ) < strtotime( '-15 day' ) ) {
		$out = "";
	} else {
		$out = '<div style="display: inline; float:right;">'
		         .'<span><img src="imageurl"></span>'
		       .'</div>';
	}
	return $out;
}
add_shortcode( 'check_if_new', 'check_if_new' );