Steve M. at No More Mister Nice Blog made a great find yesterday: there's a book called The Nightingale's Song which recounts McCain's experiences during three Christmases in captivity, 1968, 1969, and 1970. Both hilzoy and Andrew Sullivan join Steve M. in observing that if McCain's "Cross in the dirt" story actually took place, the story probably would have found its way into the book. (McCain was interviewed extensively for the book.)
Through a little deductive reasoning, we can establish that the timeframe covered by The Nightingale's Song is also the same timeframe during which McCain's story must have taken place, if it took place at all.
We know that McCain was in captivity for the Christmases of 1967 through 1972. We also know that according to McCain's account of the story, he was in solitary. That eliminates the Christmases of 1967, 1971, and 1972. (It might also eliminate 1970 -- McCain was released from solitary on Christmas Day, 1970.)
Given that McCain says he met the only guard he considered to be human in May, 1969, then we know that the "Cross" story must have taken place in 1969 or in 1970.
The fact that The Nightingale's Song covers the period during which McCain's story must have come (if it's real), but doesn't mention it at all is pretty strong circumstantial evidence that the story is in fact apocryphal.
Add to that the fact that McCain changed prisons before the Christmas of 1969 (meaning that McCain's "human" guard would have needed to follow him to a new prison), McCain's story is looking increasingly unlikely.
Update @ 10:38AM: With the McCain campaign's latest statement on this matter, it now seems that we can rule out 1970, leaving 1969 as the day on which the story must of have happend -- if it happened. Bud Day claims that McCain told him the story shortly after they reunited in prison. They reunited on Christmas Day, 1970.
© Jed Lewison