<?php
$t1=$post->post_date;
$t2=date("Y-m-d H:i:s");
$diff=(strtotime($t2)-strtotime($t1))/3600;
if($diff<72){echo "<img src='https://https://www.lilianhua.com/wp-content/themes/wp-meliora-master/ico/new.svg' alt='3天之内的文章'>";}?>

将以上代码添加到index.php文件的适合位置