Memory loss (short term/long term)

I visited my uncle in his care home this afternoon. He had two birthday cards on the table in front of him and I gave him three more. We talked about his birthday for a while before I suggested a walk to the 'library' at the far end of the corridor. (He needs more exercise.)

When we got there he looked at his watch and discovered it was the 7th. I asked him which month and he didn't know. He guessed January and when I told him it was February he was very surprised and said, 'So it's my birthday?' I asked him how old he was and he said, 'That's easy, 89, a prime number and 8 years till the next prime number birthday.'