First, the Fermat page at the St. Andrew MacTutor website
(http://www-groups.dcs.st-and.ac.uk/) gives August 17, not 20, as the
day of birth. If this isn't just a typo, where did it come from? (It
could be interesting to trace a genealogy of inaccuracies, to watch
who pulls information off MacTutor, who gets it from DSB, etc.)
Second, exactly what does it say on Fermat's tombstone? The date 1608
is consistent with the (presumably indubitable) death date of January
12, 1665, if it reads something like "he died in his fifty-seventh
year." But if it reads more like "died at age 57," then 1607 would be
the more likely inference.
Barry Cipra
Northfield, Minnesota