and adjustments and backorders

>>Hi,
>> I am developing POS (Point od Sales) system. My client request to have feature that able to view the stock balance at the particular period such as 2 months ago or may be 2 days ago.
>> How could I retrieve the stock qty at certain period? Should I re-calculate the total purchase, total return, total sales of items in order to get these stocks balance?? What do you do for these?
>>
>>Thank you
>
>Think of it as a checkbook. From the current Quantity [balance], you have to subtract any product orders received by the client from their suppliers [deposits]. You have to add the net quantity sold to the customers [withdrawals]. This means you have to have a way of tracking, by date, the orders the client places and receives that restock his inventory as well as track all sales and returns by the customers.