It's possible the date is hallucinated. There is no reason that a combination of system prompt and regular prompt when combined can not generate a hallucinated cut off date that does not match the actual date.

LLMs are statistical models and simply generate probable sequences of tokens based on a context (very much like sampling from Markov chains) so there is no a priori reason to believe that the cut off date is accurate.

More generally, all output from the model that seems to be model metadata should be assumed to be a hallucination.