First, a reserve day without being assinged duty is not rest, per se. But I know in 117 rest has to be prospective. And I'm pretty sure that its not the same in 121. Unless I'm misunderstanding intepretations. For example, in 121, when given an assignment, one has to 'look back' 7 days (168 hours) from the end of that duty and have at least 24 consecutive hours off somewhere in that time. I don't know if you are referring to something different than that?

/thread drift