Either that or you need to write an update routine to manage or automate the changes for the dates on products that are setup as products_date_available set to the future ...

When that date happens, you can have it update the products_date_added with the products_date_available and reset the products_date_available to blank ...

I'd write some custom code, myself, and maybe jazz it up a bit be able to do a few other tricks like instead of sold out or something like that have it display a "coming soon" instead of the "sold out" and perhaps an option to not allow the product to be added to the cart until the product is available ...