Update 2nd Textbox Based On Popup Calendar Selection

Question

I am using the JavaScript-based popup calendar from dynamic drive named "Xin's Popup Calendar" and it works perfectly. But, I want to be able to adjust the date of a 2nd textbox based on the selection.

For example, I want to be able to automatically adjust it to +1 month of whatever date was selected in the popup. How can I do that?

Here is an example:

<input type="text" name="firstinput" size=20>
<small><a href="JavaScript:showCal('Calendar1')">Select Date</a></small>

<p>
<input type="text" name="secondinput" size=20>
<small><a href="JavaScript:showCal('Calendar2')">Select Date</a></small>
</p>

If the firstinput date is 3/21/10, I want the secondinput to change to 4/21/10 at the same time.