I have a html form like this :

```
<form action="https://www.123.com/cgi-bin/action" method="post">
<input type="hidden" name="item_name" value="Product Name">
<input type="hidden" name="item_number" value="Product Name_2010_12_21_15_03">
<input type="hidden" name="amount" value="29.99">
</form>

```

How to use javascript to replace the above "Product Name\_2010\_12\_21\_15\_03" with dynamically generated time string ?