What I posted in my Yahoo league's "Commissioner's Note:"Some may be wondering about the Oakland-Boston series (two games) that starts in Japan on Tuesday, and how this would affect scoring for us. These two games alone do NOT count as "week 1;" the first fantasy week is officially March 24th - April 6th.
This obviously creates two extra real-life games (in "week 1") for managers starting A's and Red Sox, but this evens out throughout a long season (i.e., they play two less games the rest of the way). Between Tuesday, March 25 and Sunday, April 6: Oak/Bos have 7 games; the rest of MLB (?? at least those I have checked), 6 games.
That being said, your league starts when you set it up as commissioner. So if it week 1, then you will start DURING week 1, but for games occurring AFTER your draft (i.e., all the games beginning Monday). They will not apply retroactive stats in any situation. I am skeptical of the following, doubt it happened (if it was Yahoo):
A couple years ago we drafted after the first Japan game (Posada had 2 HR or something). All the stats that already occurred counted for our league: If you drafted Posada you also got his 2 HR. This was a roto league.