Ok I re-read your post now I get it better.
What you need is a separate table occurrence for your "shipping history" table that is keyed off of the date you input in your new table using a greater than relationship. Your calc will read something like:
sum(ShippingHistoryLookup::Quantity)+CurrentInventory
Now what happens if you have more inventory come in after the date you are looking up? You can add another TO to your receiving history with the same relationship and your calc will look like:
sum(ShippingHistoryLookup::Quantity)+CurrentInventory - sum(ReceivingHistoryLookup::Quantity)
Again I think this will get very slow in the long run. To make your approach work you will have to use an unstored calc. An alternate way of doing this is to have a separate "inventory log" table that has three fields "Date", "ItemID", and "Quantity". Whenever an item is shipped/received you just add another record to the log. This will be much faster in searches in the long run.