If all of the batman chronicles are things that actually happened then what about Batman Beyond? How can we be talking about things that actually happened that haven't happened yet? It's just plain nonsense!Well, this pseudo-anonymous denier (whose name rhymes with "Hawkins") has forgotten one important aspect about our universe: time travel. In fact, this is clearly outlined in the Batman Beyond chronicles. During Batman's confrontation with Chronos we not only learn of the possibility of time travel, but in fact the existence of a working time-machine. It is all laid out in the chronicles, but since deniers never bother to read what they criticize, this clear and undeniable fact often escapes them.
In fact, we know that Batman visited the old west while pursuing Chronos, which gives a first point of contact for information to bleed through from the future. Granted, this doesn't explain how later entries into the chronicle made it back through time. However, we have sufficient facts at our disposal to draw a conclusion.
We know that time travel is possible from the chronicles, and we know that Batman went back in time at least once. We also know that Bruce Wayne, McGinnis's predecessor, is a crafty bastard who does whatever it takes. So upon Batman's discovery of time travel, he likely decided to make use of time-travel to ensure Batman's defeat of his many enemies. So it is likely that Wayne sent a copy of the chronicles back in time in order to be able to make use of them in his time. It is even likely that he received copies of the chronicles when he was younger, spurring him to send them at a later date to avoid a paradox.