Does anyone have a dementia person who can't tell time but is somewhat clear on other things?

My father has dementia. He was a professional person and was always punctual and very aware of time. He will say I go to dinner at 10 meaning 4:50. He gets up at night and tries to get dressed for breakfast because he’s been in bed for an hour and wakes up and thinks it’s morning. I have a digital wall clock on his wall which gives the time, day and date. He has an alarm clock by his bed and he also wears a wrist watch. When he gets up he insists that it’s morning. I don’t know how to help him.