In Chapter 24, on his way across the Pacific Ocean, Passepartout congratulates himself on keeping his watch on London time because it now matches perfectly with the timepieces on the ship. The narration uses dramatic irony to foreshadow the end of the novel:
Passepartout was ignorant that, if the face of his watch had been divided into twenty-four hours, like the Italian clocks, he would have no reason for exultation; for the hands of his watch would then, instead of as now indicating nine o‘clock in the morning, indicate nine o’clock in the evening, that is the twenty-first hour after midnight,—precisely the difference between London time and that of the one hundred and eightieth meridian.
In this passage, the narration lets the reader in on what Passepartout doesn't know: his watch matches up with the time on the ship's chronometers because it only has numbers 1 through 12 and does not note whether it is nine o'clock a.m. or p.m. Halfway around the world from London, there is exactly a 12-hour time difference that Passepartout forgets to consider.
The dramatic irony here (the reader sees something Passepartout does not) creates suspense. The stated point of Fogg and Passepartout's journey is to travel around the world at an unprecedented pace, so time is of the utmost importance. The reader is left wondering when Passepartout will realize his mistake. The narrator saves the discovery for the end of the novel, when the reader has all but forgotten Passepartout's mistake. The reveal that Passepartout and Fogg have accounted incorrectly for the time change, and that they have in fact made the journey faster than they realized, is all the more dramatic because of the foreshadowing in Chapter 24.
Dramatic irony in this passage also contributes to the novel's critique of imperialism and Western attitudes of superiority over the entire world. Earlier in the same passage, the narration explains that Passepartout's mistake about the time is made possible by his condescending Western attitude toward the regions he has been passing through:
It will be remembered that the obstinate fellow had insisted on keeping his famous family watch at London time, and on regarding that of the countries he had passed through as quite false and unreliable.
Passepartout has kept his watch set to London time because he has a poor opinion of non-Western countries' ability to keep time. Ironically, though, he is the one who is wrong about the time. Fogg's hypothesis is that imperialism and industrialization (Western exports to the rest of the world) allow him to manage his schedule down to the minute, even factoring in chance delays. But here, Passepartout's strict Western loyalty to his watch and to London's time zone makes him worse at managing time. This moment foreshadows not only the plot twist at the end of the novel, but also the novel's ultimate conclusion that Fogg's hypothesis is only weakly supported by his journey.