Possessive Form

DANG it... NOOD awreddy?

Anyway...!

J.J. Sefton: It's Davis' birthday, not Davis's.

Um, officially...

Form the possessive singular of nouns with 's.

Follow this rule whatever the final consonant. Thus write,

Charles's friend
Burns's poems
the witch's malice

This is the usage of the United States Government Printing Office and of the Oxford University Press.

Exceptions are the possessives of ancient proper names in -es and -is, the possessive Jesus', and such forms as for conscience' sake, for righteousness' sake. But such forms as Achilles' heel, Moses' laws, Isis' temple are commonly replaced by

the heel of Achilles
the laws of Moses
the temple of Isis

Strunk, Elements of Style
http://www.bartleby.com/141/strunk.html