Calculating rest in retrospect is also illegal. The FAA's legal branch has been consistent in their replies to various requests for interpretations. For it to count as rest, it must be prospective ie planned ahead of time, and must be free of all duty. Doesn't matter if it's within 24 hrs, weekly, fortnightly, monthly or whatever. 'On call' is not free of duty. It's not necessarily 'duty', but it is not 'rest'.