If you have some time to kill and a lot of patience you can pick your way through this:
http://www.johnkerry.com/about/john_kerry/military_records.html
The short story is that kerry was assigned to the USS Gridley from 3 Jan 67 to July '68. He spent those months floating around miles off the coast, but according to him, that's one tour "in" Viet Nam. Guess that explains how he got to be "in" Cambodia.
Kerry's second "tour" Was from late Nov '68 to mid March '69 - a total of about four months. He counts that as his second tour.
The difference between a fairytale and a kerrytale? One start's with "once upon a time" and the other starts with "This is seared into my memory".