How to "Pre-Write" to DB Fields

Using eCart to set up a BookFair site for my daughter's school Parents Association. I have done the site for previous years using PHP/MySQL snippets I gathered and cobbled together, but I want to make some of the functions more robust, so I'm recoding using DataAssist and eCart.

The primary db is the 'books' db, where all the pertinent book data (including donor info) is stored. There are two fields that determine book availability -- reserveDate and purchaseDate. These are integer fields filled with timestamp info. If the reserveDate is less than 30 minutes the book is noted as "Reserved" in the wishlist display. This value is checked every time the wishlist page loads -- if it is more than 30 minutes old it is reset to 0.

My current problem (I'm sure I'll find more as I move forward!) is figuring out where to place the code that resets the reserveDate to 0 for all books in the cart if the cart is cleared, and also how to reset it should the Delete Item function be used.

Suggestions?? I can supply any specifics needed. . .

Thanks,

Mike