The thing is, I don't think what you tie the transitions to, has much meaning from a technical standpoint, beyond the why you'd choose to. We've seen that you can view the transitions from anywhere on the track as a result of the replay mode. The main limitation with FM7's time of day is that it can only transition within a limited range, as a result of the baked lighting/shadows. Due to this, having the transitions be tied to time makes less sense, because no matter how much time passes, it would never be able to progress past the range of time it supports. Race progress however has a defined start and end, that you can map the range of the time passage to, which would never be exceeded.
EDIT: Or not, if there's no free cam in replays.