Modifications of the warehouse inventory script

We have a php script for management of financial and sales data (to collect and compare the data from our companies located in different countries). We would like to modify the script as follows: 1. In 'inventory' section of our website users should be able to add products inventory only on Fridays (and not other days of the week) and on 31st of December. 2. In the same section, when searching for example inventory of selected country on 7.12.2007, products should be sorted based on their numeric position set in Product details page. 3. We would like to avoid users to add the same product several times in the same day (example: some users add Product xy 2 or 3 times on the same day). We want they are able to add details on stock of (for example) Product xy only 1 time on Friday (or to edit that record).