enableHeartBeatTimer The heartbeat timer Settings did not take effect

_paq.push(['enableHeartBeatTimer', 15]);

  _paq.push(['trackPageView']);

  (function() {

    var u="//matomotest.china.cn/matomo/matomo/";

    _paq.push(['setTrackerUrl', u+'matomo.php']);

    _paq.push(['setSiteId', '1']);

    var d=document, g=d.createElement('script'), s=d.getElementsByTagName('script')[0];

    g.type='text/javascript'; g.async=true; g.defer=true; g.src=u+'matomo.js'; s.parentNode.insertBefore(g,s);

  })();