I am really tired just having returned from my second driving trip in two weeks, and a very hot-weather 3-day cowboy shooting match. So, forgive me if I missed an answer to the inquiry of why manufacturers use a date code, instead of just dating the ammunition with a clear date.
Remember, this ammunition is for commercial sale. Most people do not realize that quality ammunition, properly stored, remains perfectly good for decades, not just years. If an average “shooter” customer (as opposed to a real student of arms and ammunition) went into a gun shop and was handed a box of ammunition dated seven or eight years earlier, they might well refuse this “old stock.” Factories, though, need some control over their product in case of malfunction, legal claims, etc. Some use a lot number, some use a date code and some use both. Many lot numbers, if not all, have a date code built into them, often kept very secret by the manufacturer.
While we had a large and fast turnover of ammunition product in our store, since we stocked almost every caliber available in factory ammunition, there were, of course, some calibers that didn’t sell as quickly as others. It would not have been impossible for us to have a box of ammo 10 years old on our shelf. Believe me that no one gets short-changed with a box that old from a dealer that stores his ammunition properly. Some surplus ammunition can be 50 years old and still perform well. That has been the case, for example of the Austrian (German-Occupation) made 8 x 56R ammo that has appeared off and on on the market in recent years. Most of that ammo is dated 1938 - now 69 years old!
So, date codes are simply a selling device to keep from “turning off” customers with the age of product. However, again I’ll stress, the presence and interpretation of that code has much more significant importance for the manufacturer than just a “sales gimmick.” The advantage of the code above a plainly-read date is primarily for the jobber and the retail dealer.