Too much trouble. The next script snippet will show 18:06 as time.

alert(new Date(2005,1,8,17,66,00));

Note that I specified 66 as the minute parameter. The Date() function is able to handle overflows perfectly.

Adding 5 minutes:
<script>
var now = new Date();
alert(new Date(now.getYear(),now.getMonth(),now.getDate(),now.getHours(),now.getMinutes()+5,now.getSeconds()));
</script>