How to make jquery count down timer function manually editable

You can achieve this using wp_footer action. Add bellow code to functions.php file. It will load the script at the footer of page.

add_action('wp_footer', 'load_count_down');

function load_count_down(){
    if ($year = get_option('of_year') && $month = get_option('of_month') &&  $day = get_option('of_day') ) {

        $date = $year."https://wordpress.stackexchange.com/".$month."https://wordpress.stackexchange.com/".$day;
        $script = <<<EOF
<script type="text/javascript">

var siteCountDown = function() {
    if ( $('#date-countdown').length > 0 ) {
        $('#date-countdown').countdown('$date', function(event) {
          var \$this = $(this).html(event.strftime(''
            + '<span class="countdown-block"><span class="label">%w</span> weeks </span>'
            + '<span class="countdown-block"><span class="label">%d</span> days </span>'
            + '<span class="countdown-block"><span class="label">%H</span> hr </span>'
            + '<span class="countdown-block"><span class="label">%M</span> min </span>'
            + '<span class="countdown-block"><span class="label">%S</span> sec</span>'));
        });
    }
};
siteCountDown();
</script>
EOF;

    echo $script;
    }
}