All of those questions can be answered through comments. I'd vote for a separate function. Some of the answers can be encoded in the type system through the us

All of those questions can be answered through comments. I'd vote for a separate function. Some of the answers can be encoded in the type system through the use of time units (like std::chrono), optional return values and design by contract (preconditions). Unit tests could be added to further clarify some edge cases.