The GetSelectedRow in WebCombo javascript function will return a HTML element. In order to access the text in the cell you will need to use the getElementsByTagName javascript function.  Here is the snippet to access the third cell text in the WebCombo selected row:

function getComboData() {
    var combo = ISGetObject('cmb');

    var selectedRow = combo.GetSelectedRow();

    var dateText = selectedRow.getElementsByTagName('TD')[2].innerText;

    alert(dateText);
}

Since the dateText will be a string type, you will need further process to convert it to Date object for your scenario of comparing 2 Date object

Brilliant. Thanks for this post. It was exactly what I needed!