Finding Oldest Date

date1/Jan2/Jan3/Jan4/Jan5/Jan6/Jan7/Jan8/Jan9/Jan10/Jan11/Janquantity 502023108948202535current stock 200oldest date of manufacture?

I have a report that lists the quantity of an item that is manufactured on particular day. On some days there will be no items produced. The report is similar to the above table. I don't want to change the table format.

I know the current stock of that item (in this example it is 200). Note that the current stock is not just the sum of the quantiy for each day's production.

I need a function that will determine the oldest date of manufacture of material in the current stock. The function assumes that that the current stock is made up of the most recently produced material. ie the stock is depleted on a first in -first out basis. Essentially the calculation has to sum backwards from the most recent manufacture date until the total = 200 and then lookup to corresponding date.

In the above table the oldest prodcution date would be 6/Jan. The answer to the funstion in example = 6/Jan