"One of these things is not like the others."

[stoutfellow]

Prompted by a recent retro-review by James Nicoll, I decided to reread Alexei Panshin's Villiers trilogy. The first book, Star Well, begins as follows:

To history buffs, the year was 4171 A.U.C. To Christians, it was 3418. To Moslems, it was late in the year 2795. But by common reckoning, the year was 1461.

Two of those four numbers are certainly consistent. Another is consistent with them, given an assumption which at the time (A.D. 1968) might have seemed plausible. The fourth, however, is flatly wrong - utterly inconsistent with the other three.

Challenge: which one, and why? I'll be screening replies (on the off chance that there are any), so as not to spoil anybody, and I'll explain the problem later.