How to represent a CarePlan template with relative doses and timings?

I’m working with data involving templates for CarePlans, which have doses that have to be adjusted based on the patient weight (so, e.g., their units could be in mg/kg) and whose scheduled times of administration have rich structure but need to be relative to when the patient actually starts treatment (which is TBD).

How could this be represented in FHIR? We’ve looked at OrderSet, which seems promising because there are fewer irrelevant patient-specific fields, but even then there doesn’t seem to be a way to indicate relative time structures.

The intention here is to have an extension on the date and dateTime data types that allows expression of relative time (so time offset (positive or negative) and reference (codableConcept and possibly a reference to another resource instance) that defines what the offset is from. However, we haven’t gotten far enough in the workflow process to define that yet. The extension would then appear in protocols, plans, order sets, etc.

We also plan extensions that allow elements such as dosage to be expressed as calculations, though you can express something like 5 mg/kg using the existing Quantity data type for simple things like that.