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//HTML
<span id="date-date">
  20160818
</span>

//JavaScript
var dateStr = jQuery("#date-date").html();
var match = dateStr.match(/(\d{4})(\d{2})(\d{2})/);
var betterDateStr = match[2] + '-' + match[3] + '-' + match[1];

jQuery("#date-date").html(betterDateStr);