They're making a movie about clocks. It's about time.
Why this joke works
A Synopsis That Sounds Impatient
“It’s about time” can express impatience that something has finally happened. Here it also gives a perfectly literal description of a film whose subject is clocks. The second sentence sounds like a reaction to the announcement and a miniature synopsis at once. Give the opening the tone of a casual entertainment update. Then let the final sentence carry mild relief, without adding finally to explain the phrase beforehand.
A closer look
A Closer Look
This differs from a movie-length joke: the word about still concerns a subject in the literal reading, while the whole phrase supplies the impatient idiom. A flat delivery favors the synopsis; a relieved delivery favors the overdue announcement. Either works, but a slight change of tone lets both interpretations remain available.
Fact behind the joke
Fact Behind the Joke
Cambridge lists high time as an alternative to about time when something should have happened sooner. That substitution preserves the impatience but would not describe a film’s subject. Comparing the two expressions shows how the original happens to allow a literal synopsis alongside its established conversational meaning.