If your race date is May 13 and your plan lasts 20 weeks, you would back out 20 weeks from May 18 and start your plan on Dec. 25.
But I'm not sure that's what you are asking...?
Transition Tips for a Half Iron-Man?
Full and half distances in FW, May 2017
Half Ironman or full marathon - which is harder? Pages: 1 2
Anyone used Don Fink's 30 week intermediate program? Pages: 1 2 3
Full sleeve vs. sleeveless wetsuit