Keaton uses the iris effect to move the story line, iris's on a CU of the letter detailing 'must die Sep. 1st', following TT's point, iris's in on the calendar reading 'Sep. 1st.' guiding audience attention so they follow the story line.
He builds this further with MS of Mr NN, iris effect continued, reading the letter warning him: Keaton using objective camera work to story tell.