Doc is correct, the date on the case has nothing to do with the date the rounds were loaded. I have more than 50 German WWI boxes of The 9mm P08, and almost all have mixed dates on the headstamps. Usually, they are from the two or three months earlier than the date on the box, but sometimes considerably longer.
I have been told that before WWI Germany stored cartridge components, particularly rifle cases and bullets, in a strategic reserve to be loaded in event of war. These cartridges were periodically replaced so it is likely that some rounds from this period were loaded a number of years after the cases were produced.
The most extreme example I have is a Polte 9mm P08 with a brass case dated 1937 loaded with a black mE bullet which has been describe as an early experimental mE load. That is not true since in 1937 the mE bullet was just beginning to be developed. The earliest known drawing is from Oct 1937 and it is called an MK bullet (IAAJ #455 or gigconceptsinc.com).
Eventually, I found a full box of these rounds which correctly identified the cases as 1937 Polte manufacture, but loaded by emp in 1943! Polte stropped producing 9mm cases with lot 34 of 1942. Apparently they later discovered this stash of 1937 brass cases and shipped it to emp for loading.
As Doc indicated above, the headstamp may tell you very little about who loaded the cartridge or when.