I saw The Trouble with Harry (1955) a while back and it didn't have a big impression on me. But recently I rewatched it and was amazed at how odd time seems to flow, how somewhat confused people are, as if they have memory problems.
This review goes into detail about the movie, but I wanted to focus on a single riddle in the movie. It's this exchange with the kid:
Arnie: Why haven't you visited before?
Sam: Perhaps I'll come back tomorrow
Arnie: When's that?
Sam: Day after today.
Arnie: That's yesterday, today is tomorrow.
Sam: It was.
Arnie: When was tomorrow yesterday, Mr. Marlowe?
Arnie: Oh, sure. Yesterday.
Arnie seems to be the only one that remembers reliving days. Every adult just has slight confusion at times; forgetting things or having unexplained familiarity with each other.
We learn from this exchange the loop is not identical (Sam never visited before). But time does not flow properly since Arnie has learned not to expect that tomorrow will just come the next day ("When's that?").
Actually, Arnie knows that the day after the present day is... "yesterday"; the loop restarts. If anything it looks like the present day is something entirely new to Arnie. It's finally a "tomorrow".
My impression is that the loop had a single day until now. It was "yesterday" on a loop. The present day is finally a "tomorrow".
Sam and Arnie obviously talk about things from different perspectives. Maybe Sam has a slight intuition about things (being an artist) but only Arnie knows about the loop. So, it's not clear what Sam means when he says "It was".
Anyway, this puzzles Arnie which asks "When was tomorrow yesterday, Mr. Marlowe?"
Now, Sam answers "Today". This may just be related to the talk they had: Arnie mentioned that the day after today is yesterday so, today is the day when yesterday will come tomorrow.
But then Arnie thinks about the question he just asked and finds his own answer: in the previous loop of yesterday tomorrow was always yesterday.
It almost looks like the timeline was: Yesterday1, Yesterday2, ..., Yesterday N, Today (brand new).
Based on the ending we know the adults intently do a time loop: leave Harry in the forest for Arnie to find again.
So, the timeline is: Yesterday 1, Yesterday2, ..., Yesterday N, Today1, Today 2 (new loop).
It does not look to me that time is going in reverse. It looks to me like there's a daily loop that doesn't progress until things are settled.
Going back to the missing piece: Sam says "it was" suggesting today was tomorrow. Assuming this is the 1st day in the new loop, it does make sense indeed: today was yesterday's tomorrow, but since they are entering another loop there will not be another new day, just today on repeat.
The riddle is interesting because of there's no language to express time loops properly. "Tomorrow" may mean the day following the present day which may be a re-run or loop or "tomorrow" may be the day naturally following current events. A more proper (but dry) phrasing would be:
Arnie: Why haven't you visited in any of the (repeated) days I remember?
Sam: Perhaps I'll come back tomorrow.
Arnie: When will we get to live tomorrow?
Sam: Day after today.
Arnie: After today we will relive yesterday, today is a new day following the yesterday loop.
Sam: But the next day won't be new.
Arnie: When did we live the day before yesterday, Mr. Marlowe?
Sam: [Nonsensical response.]
Arnie: [Gets confused.] Oh, sure. We continuously relived yesterday.