You know, since this has all started, I've been trying to think of a way to make some of this easier for you. Right now you're process is, apply an expected due date, leave the date alone until the product arrives at which point you have to manually go and clear that date... This is a smidge of an override of the way ZC works as a default... The problem with this override is what if you have say an event or something that you "refuse" to allow being sold until a certain date (publisher prevents release until for example).... Now though you are chomping at the bit and have to manually login at "midnight" to do that mod...
If though you had an addtional piece of data, say an on/off switch that when off, then all this additional logic applied, but when on (default for example) then the date auto-cleared and the product was "magically" available, it seems like it would make the best of both worlds...
Part of the reason I suggest this also, is because it could be relatively easy to "add" to the cart. Check this thread out:
https://www.zen-cart.com/showthread....inued-products
The code provided there (though on a todo list for me to take a little further) would offer a "button" that could be clicked from the admin products listing, the effect of being a controller for using the modified code in one state and the default code in the other... Anyways, the todo part is to incorporate it into the product_info sections as well... Rather than just on the products listing page.