Contrary to popular belief, a name ending with an ‘s’ does not warrant the ‘s’ be included as a part of the name’s possessive form. According to The Elements of Style by E.B. White,
“Form the possessive singular of nouns by adding ‘s. Follow this rule whatever the final consonant. Thus write,
‘Charles’s friend
Burns’s poems
the witch’s malice’”
(emphasis added). Thus, the above usage, “…Timmons’s…”, is correct.
(For Math 381, Winter 2007)