Here is a delivery date module that will let the user enter in a del. date. It will not calculate $ or do anything but get the date from the customer AND you can disable dates. Its pretty neat, and easy to install... google this:

Contribution: Order Delivery Date
Version: 2.2
Designed For: Zen Cart v1.3.8a Release
Forum Page: http://www.zen-cart.com/forum/showthread.php?t=92762