I have talked to my GME office multiple times about this. And the honest truth is, they don't really know and it always gets confusing. But, as far as I can tell, what's posted above is not correct. I think the post that turqoiseblue is accurate, but his (and WS's) interpretation of what I said is not correct.
Here we go:
1. Every year that you do of training "counts" towards your maximum funding, regardless of prelim / chaging fields / etc. Hence each year you train counts as a year of training.
2. If you start with a categorical program, then your clock is "set" to expire when you use the minimum number of years required to train in that field. This sets the maximum number of years you can get funded.
3. If you start with a prelim / TY year, then your clock is not set. That happens when you start some other, "terminal" program that will result in board certification. However, when that happens all of your prior prelim training "counts" against that clock.
4. If you match into a field but never start it, then nothing happens. You actually have to train in that field to have your clock set.
So, examples:
1. A student matches into and starts a Categorical IM program. Therefore, his clock is set at 3 years. He completes three years of training and decides to continue in an Endocrine fellowship. That fellowship will be funded at 50%. DME.
2. A student does a prelim medicine year (1 year), followed by a radiology residency (4 years). During the first year, one year of funding is used but the clock is not set. During the second year, clock is set at 5 years (since that's how long it takes to train in rads including the manditory prelim year).
3. Same student in #2 decides to resign her rads spot prior to starting, had a change of heart and wants to go into IM. Crazy, I know. She starts as a PGY-2 in IM. Clock gets set to 3 years when she starts the IM residency, and the prelim year counts as 1 year against that maximum. Matching into rads does not set the clock.
4. Same student in #2 decides to resign her rads spot after 1 year of rads training. Switches to IM, again as a PGY-2. This time, the clock was set to 5 years once the rads training begins. 1 year used as prelim, 1 as rads. She can finish her IM residency in another 2 years, and then has one full funded year left if she decides to do a fellowship.
How CMS keeps any of this straight is beyond me.
And, just to make things even crazier, Medicare uses the Oct 1 - Sept 30 accounting cycle.