In reality, it seems to be down to "what the primary letter-writing therapist defines that way." In my case, due to wholly unrelated legal issues, I was full-time including at work by June 2010, but couldn't change my name for six months after that. My therapist said she considered coming out at work to be the start date regardless, but she didn't exactly demand legal proof or anything; she took my word for things.
In the end it didn't matter, since I got GRS about two years past that point. (Jill : I don't think that's odd at all. In fact, GRS was my first and ONLY surgery! I would have been happy to do it at the precise 12-month mark, too, but we had to save up some more money first.)