How to detect idle time in JavaScript

Here is a simple script using jQuery that handles mousemove and keypress events.
If the time expires, the page reloads.

<script type="text/javascript">
    var idleTime = 0;
    $(document).ready(function () {
        // Increment the idle time counter every minute.
        var idleInterval = setInterval(timerIncrement, 60000); // 1 minute

        // Zero the idle timer on mouse movement.
        $(this).mousemove(function (e) {
            idleTime = 0;
        });
        $(this).keypress(function (e) {
            idleTime = 0;
        });
    });

    function timerIncrement() {
        idleTime = idleTime + 1;
        if (idleTime > 19) { // 20 minutes
            window.location.reload();
        }
    }
</script>