What are the discrepancies between Verne's Twenty Thousand Leagues Under the Seas and The Mysterious Island?

Question

Twenty Thousand Leagues Under the Seas, first published in 1871, is one of Jules Verne's best-known novels. In 1875, Verne published a sequel, The Mysterious Island.

The Wikipedia article about the first novel has a section about Recurring themes in later books which contains the following statement (emphasis added):

(…) Hetzel and Verne generated a sequel of sorts to this novel: L'Île mystérieuse (The Mysterious Island, 1874), which attempts to round off narratives begun in Twenty Thousand Leagues Under the Seas and Captain Grant's Children, aka In Search of the Castaways. While The Mysterious Island attempts to provide additional background on Nemo (or Prince Dakkar), it is muddled by irreconcilable chronological discrepancies between the two books and even within The Mysterious Island itself.^{[clarification needed][citation needed]}

What are these discrepancies between Twenty Thousand Leagues Under the Sea and The Mysterious Island?

Answer 1

TL;DR:

• TMI is set in 1866-69.

• Castaways/Grant is set in 1865, less than 2 years earlier. But Ayrton states in TMI that he was abandoned 12 years earlier = 10 years discrepancy

• Main events of 20,000 Leagues happen in 1867, 2 years before meeting Nemo in TMI!. But Captain Nemo is an old man who claims those events happened 16 years before = 15 year discrepancy.

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The blog post Jules Verne’s Calendar Problem by Steven R. Southard (July 2020) explains the time differences in detail, even providing the diagram:

In Verne’s novel In Search of the Castaways (also called Captain Grant’s Children), the main characters abandon the traitorous Tom Ayrton on a deserted island in March 1865.

In the subsequent novel Twenty Thousand Leagues Under the Sea, three main characters are taken aboard the Nautilus in November 1867.

So far, so good. However, in The Mysterious Island, the castaways find Ayrton in December 1866. Ayrton states he was abandoned 12 years earlier, in March 1855. (Not just less than 2 years, as simple subtraction would indicate.)

…

Later in The Mysterious Island, in October 1869, the castaways come across Captain Nemo. He states it has been 16 years since the three guests came aboard the Nautilus. (It had been just shy of 2 years, but maybe time moves slower on that island.)

Please note that Jules Verne acknowledged the timing problems (the same post quotes Verne and publisher's explanations).

The post author explains the discrepancies:

It’s a strange attempt at chronological hand-waving, but we see what happened. Verne’s proclivity for including precise dates in his novels got the best of him. After publishing Captain Grant’s Children, he wished he had set that novel ten years earlier. That way, Ayrton would have been living alone for 12 years rather than 2, and more believably reduced to an uncivilized state.

Similarly, Verne needed a much older Captain Nemo in The Mysterious Island, an aged and lone survivor of his crew in 1869. Only then did Verne wish he’d not already written about a younger and energetic Nemo, and full crew, set in the years 1867-8.