How to add 1 year to selected date?

How to add 1 year to selected date?


I'm trying to add 1 year to a textbox from a date picked by and end
user can someone help me with this?
Currently this is what I'm using but its just appending 365 to the end
of the date...
<script type="text/javascript">
    $(function() {
        $("#vis_los_date").datepicker({changeMonth: true,changeYear: true,
            onClose: function(set_date) {
document.getElementById('los_date').value =
document.getElementById('vis_los_date').value;
        }
        });
        $("#pol_effdate").datepicker({changeMonth: true,changeYear: true,
            onClose: function(set_date) {
document.getElementById('pol_expdate').value =
document.getElementById('pol_effdate').value+365;
        }
        });
        $("#pol_expdate").datepicker({changeMonth: true,changeYear: true});
    });
</script>