Auto populating an exp.date, keying from 2 variables

Hello all,

I've set up a form for my CSR's while processing orders and I'm stumped on this one last item. Here's a brief set-up:

A drop down menu selection is Automatically chosen once the page is loaded. This is keyed from the customer's registration. A couple examples from the drop down menu-

XX-3MO
XX-6VC

(XX being the service package, 3MO being a 3-month money order and 6VC being a 6 month Virtual Check)etc, etc.

This is what I want to accomplish:

When the CSR types in the Billing Date, I would like for the Expiration Date to be populated upon hitting the 'Tab' key.

For example:

A CSR creates a new account and the following Package is already selected:

XX-3MO

... the CSR then types in the 2-digit billing date in the next field down:

09

... at this point the CSR hits the 'Tab' key. How can I get the scrip to reference both the Package and the Billing Date to return(in this example):

2002/04/09

Can this be done? I am truly sorry that this is so long winded, it's just driving me crazy!!! Thanks to all. This is a great site.