As noted above, rest must be prospective, or known in advance, but it must also be rest. The mere fact that you didn't fly for ten hours is not enough. Sitting at attention in the FBO waiting room for 10 hours, for example, is insufficient. You must be free of all duty to the company. If you're out of contact from the company, in a hotel room, that's fairly straight-forward. If you're sitting in the airplane, waiting, it's not a particularly compelling case that you're free of all duty, or free from the influence or interruption of the company.

Far too many 135 operators have tried to say that the pilot didn't fly for ten hours, and it's therefore rest. I worked for a 121 operator that did that, too. We put the hurt on that practice, however, and it proved expensive for the operator.

Ultimately, if you don't get the rest that's required, the FAA may lean on the employer, but they'll definitely break you. Keep that in mind. It's your certification and your career. Your safety, too. Protect it, accordingly.