Hi Julia - at the moment, Proof does not look at the schedules in Workfront so doesn't have the info.
However, I know the product team has been researching for some time and looking to implement later this year (potentially) a solution where the proof workflows are living in Workfront itself. Once in Workfront, it should be able to read the schedule attached to the project and count Holidays.
Obviously, this is just a hypothesis, but getting the workflows for proofs in Workfront and integrating the tool more will be really helpful to achieving this dream!