I think the 12 month validity of the ticket runs from the date of the first flight and not from when the ticket is issued but perhaps others can confirm that (as I've never done a ticket that I wanted to go for more than 12 months out from when it was originally issued).
Have just seen this question, and
Wasabi Tofu has already confirmed that this is indeed the case.
I just wanted to add that anyone planning this should be prepared for quite a bit of hassle from some OW carriers in the rare case of using a paper ticket. Check-in staff see so few paper tickets that one of them that might be out-of-date can definitely pose problems. A little over a year ago, I resumed an RTW after a long break holding a paper ticket for a 20-segment AONE5 out of CPT. My experiences were quite educational.
CX kept me standing around for about 30 minutes at the F check-in in HKG whilst they checked with AA. (For those who don't know, this is a lectern-style check-in.) I was literally left standing there for all that time, and there were no seats whatsoever. Hardly F treatment when I should have been in the Wing enjoying myself.
There were similar problems with BA in GVA, and, in NRT, JL staff even come into F on the a/c with questions
and met me on arrival in HKG because of it. To give them their due, neither QF nor AA batted an eyelid, and other BA & CX flights were OK, except that at BKK CX triple checked the taxes but never questioned the date.
Just to fill in, the ticket had an issue date in April 2008 & I was flying in Jan & Feb 2010. Perfectly legal by the rules, as my initial flight out of CPT was 28 Feb 2009, but the hand-written (by Cindy Jacobus) ticket
appeared to be a bit long in the tooth to many people.