If I'm understanding it correctly, what this sort of approach will give you is a timestamp in the prompt which is generated when the prompt is printed to the terminal and then doesn't change.

The thing is that the prompt might sit there for minutes or hours before you use it to run a command, so that timestamp doesn't reliably serve as an accurate record of when a command was run.

That's the problem which my approach addresses by updating the timestamp when a command is run.