Sandbox

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.10.6/moment.min.js"></script>
ETA to 13:00 : <span id="eta"></span>

JavaScript

function getEta() {
    var n = new Date();
    var end = new Date(n.getFullYear(), n.getMonth(), n.getDate(), 13, 0, 0, 0);
    var a = moment(end);
    var b = moment(n);
    var diff = a.diff(b);
    var eta = moment(diff).format("H:mm:ss");

    $('#eta').text(eta);
}

// First launch
getEta();

// Recalculate every seconds
setInterval(getEta, 1000);