Confirmed by introspection
The Memory Accretion Hypothesis is confirmed by introspection. Ask youself 'why do I think there is a past?' It is because you remember it. We also remember that the stock of memories is constantly being added to. Furthermore we infer that the process will continue, leading to belief in the future.
Recourse to introspection can be controversial in philosophical and psychological debates. Some - such as Descartes - thought that it provided infallible insights ('I think therefore I am'), while others dispute this. For our purposes it is appropriate since in this part of the map we are explicitly concerned with phenomenal aspects of time perception. Indeed it is difficult to see how this kind of claim can be supported other than by appealing to introspection.
I - the person who added this node - am presuming that you - the reader - will share this introspective revelation. Why do you think there is a past, that is being constantly added to? If not because you remember it, then please feel free to challenge this argument.