As I said on my previous message, what the export gets is a current snapshot of your database and in your database, are not saved historical values of your products' stocks.
Perhaps it could be achieved if you export all the orders made since December 31st and sum up the quantities sold for a specific item. So, knowing the amount sold since then and knowing the current stock, it's easy to know the stock that product had at that specific date (just summing the sold items to the current stock).
Using the Action Logs add-on, you could read all the historical stock changes to any product, so you can know on a specific date (if the stock changed) which was exact value by then. But the add-on will start logging when you install it (so there won't be logs for past dates).