<script type="text/javascript">
        var start = '21:00';
        var end = '22:20';

        s = start.split(':');
        e = end.split(':');

        min = e[1]-s[1];
        hour_carry = 0;
        if(min < 0){
                min += 60;
                hour_carry += 1;
        }
        hour = e[0]-s[0]-hour_carry;
        diff = hour + ":" + min;

        alert(diff);
</script>