Answer by David Dickinson: The day you receive the claim, it's "Day 0". The next day is "Day 1". No different than your birthday or anniversary.
Answer by John Burnett: I feel the need to add here that the OCC was of a different opinion on this question, but I don't know whether it has returned to the mainstream to agree with David (and me), or continues to treat the day you receive the claim as "Day 1" in the count. If you work for a national bank or federal savings association, I suggest you check this question out with your lead examiner.