Surprisingly, it's quite difficult to answer this question. As dizzlmcwizzl mentioned, what counts and what doesn't count is up to individual districts. In general:
- mixed counts into it's own M rating. A player will only get this rating if he/she played mixed exclusively... or more precisely, if player played no more than two (or maybe three?) "eligible" non-mixed matches.
- tournaments count in some districts but not all. In districts where tournaments count, results from Age, Open, etc. are included into NTRP calculations.
- there is a cut-off date and it is (I believe) Oct 31. However, in some districts (the ones with "Early Start?) the cut-off date is much earlier... around end of August?
dizzlmcwizzl, I'm curious: why do you consider T (tournament-exclusive) rating "not very helpful"? I feel it's as "valid" as C rating. I agree though that M (mixed-exclusive) is a different animal...