http://hl7-fhir.github.io/datatypes.html#1.18.0.15 has a single column called "period+units". It would be clearer to break this down into two columns or at least two distinct labels within the same column, with names that match the fields ofTiming.event (i.e. `period` and `periodUnits`). Ditto for `duration` and `durationUnits`.