As long as you're consistent with your method, I don't see why it matters how you do it. If you're going to take it as the date that you pull the raw materials from stock, then fine. If you're going to take it as the date that it's packaged, fine. As long as you're consistent with it, I wouldn't see why it matters...having a date should be able to lead you to the production records, and then you're good. (Mind you, I think it would make more sense to take the start date of the run, from a record-keeping perspective.)
I suppose it depends on what industry you're talking about as well, though. If we're talking about food, beverages, or something like that, then you may be subject to more specific rules.
"Why should I be the one to change my name? He's the one who sucks!"
-Michael Bolton, Office Space