Rolling Stock Count

Good morning,

Apologies if this is displayed somewhere else but I am confusing myself over it.

I have a spreadsheet with 1265 lines. The lines are either our current stock levels, new productions and the customer's orders. What I require is code for Excel to look at groups of items and then go through each date to make sure we have enough stock to cover future orders. I would like it to highlight where we won't have stock available (it becomes a negative value).

Any help will be greatly appreciate. Would you be able to write next to it what each part does, so I can learn for future?

Thank You