People write all sorts of things in their diaries. If the diary of a famous person was being published, the editors might cut the mundane details out.
For example, from The Flash’s diary:
Almost overslept today. I bought the wrong cereal and I didn’t want to run to the store for more. So I just ate it really fast.
Saw Clark Kent on the way to work. He was driving that little electric car he has. It’s just “save the earth, save the earth” constantly with that guy.
I tried not to let him see me, but he did. He started revving his engine at me. He always wants to race! Like his car could beat my nifty Ford Prefect. Yep, nothing like an imported English economy car.
Any more?