I don’t want to give the impression that FHIR is waiting for a new dose syntax before it can be used.
If you have a good understanding of all your own system’s data items you can map them onto the data items of FHIR now.
There are many examples of complex FHIR dosage already in the standard.
But mapping between complex models is never easy, and the new dose syntax will help if you are trying to standardise the way dose is represented across a wide range of settings and systems - many of which don’t capture that data in a structured way currently.
I do know there is a need for that work in the UK, to solve a wider problem, but I am not sure that is what the questioner is trying to do here.
To add to Ian’s answers about the specifics:
1 FHIR doesn’t calculate times. If you have that requirement you need to manage that in your own system code. However FHIR does let you represent the planned time, if there is one, and the eventual time also.
2 FHIR allows you to specify the time of the dose in the request. You can specify the actual time that it happened in a MedicationAdministration. However as we have discussed already MedicationStatement is a more general purpose, higher level summary, and would not normally cover all the times of doses. That is not its job.