Mint Slice Biscuits

So I was in Met the other day, changing some water filters and doing some other routine maintenance, when I was offered a Mint Slice biscuit. The guys there had found an old packet, and I had to give them a try. And they were awful. The photo below shows two packets - the top is the new(?) packet, and the bottom one is the old packet. Any guesses as to how old they are? The answer is in the next post down.