Разбор HTML с помощью ссылки на сущность

См. JsFiddle DEMO

    var date1 = new Date();    
    var date2 = new Date("2015/07/30 21:59:00");
    showDiff();

function showDiff(date1, date2){

    var diff = (date2 - date1)/1000;
    var diff = Math.abs(Math.floor(diff));

    var days = Math.floor(diff/(24*60*60));
    var leftSec = diff - days * 24*60*60;

    var hrs = Math.floor(leftSec/(60*60));
    var leftSec = leftSec - hrs * 60*60;

    var min = Math.floor(leftSec/(60));
    var leftSec = leftSec - min * 60;

    document.getElementById("showTime").innerHTML = "You have " + days + " days " + hrs + " hours " + min + " minutes and " + leftSec + " seconds before death.";

setTimeout(showDiff,1000);
}

для вашего кода HTML: