Add/subtract a value based on date

[jrcream]

I have an array with rental item names on the x-axis and date on the y-axis. The array contains the amount of inventory for each day. When a certain amount of an item is rented I want the array to be updated so that the amount rented today is subtracted from that days inventory but is also added back to the date of return (which is known at the date of rental).

[WinteE]

Please attach an example of your file, the information you gave is not enough.

For example: what's the set-up of the table where you keep the real rentals ?

[jrcream]

I attached the example file. Sheet1 is the Inventory, Date array and Sheet2 is the sheet used to input rented items and the length of time they will be rented for. I want the inventory at any day to be a base quantity minus number of items being rented for that day and everytime one records an item as being rented in an invoice (Sheet2) the array will be upated.

[WinteE]

Take a look at the attached file and find out about =SUMPRODUCT().

Success,
Erik