Input (+1 time CET)

href="style.css?<?php $time = date("H:i:s", time()+3600); echo $time; ?>"

Output

href="style.css?21:10:12"

Tømme cache

<!-- empty cache -->
    <meta http-equiv="Cache-Control" content="no-cache, no-store, must-revalidate" />
    <meta http-equiv="Pragma" content="no-cache" />
    <meta http-equiv="Expires" content="0" />
<!-- empty cache end -->

Alternativt – tilføje dato til title tag

 <title><?php $time = date("H:i:s", time()+3600); echo $time; ?> - <?php wp_title( '|', true, 'right' ); ?></title>