Why does the Immunization, Administered data model not have Relevant period, similar to Medication, Administered?

There is a special note with a different definition only for Immunization, Administered, p25 in 'Quality Data Model, Version 5.3 (Annotated Version) for CQL', August 2017

Timing: The time the immunization is administered, i.e., a single point in time, or
Author dateTime.

However, on p14 of the same document author dateTIme is defined under section 3. Timing Considerations for QDM Data Elements
... QDM uses author dateTime to indicate the time a data element is
documented, i.e., a single point in time. To assure accurate retrieval of time-related information from clinical software, QDM 5.3 includes more explicit definition of the intent of start and stop for each datatype. ... QDM 5.3 uses the term period to indicate these intervals to be consistent with current activities in HL7 Fast Healthcare Interoperability Resources (FHIR). 

FHIR does not define author dateTime as a standard attribute of it's resources.  I do see that, unlike Medication, Administered, which contains a period, the FHIR resource for Immunization, Administered contains an attribute for a single [Immunization.date||https://www.hl7.org/fhir/immunization-definitions.html#Immunization.date]

 I would recommend that QDM either

1) follow the FHIR model and include a distinct field with the administration dateTime, separate from author dateTime, or

2) align both FHIR and QDM to include a relevant Period for Immunization, similar to Medication, Administered.  

While it is true that Immunizations are typically injections at a single point in time, they can be represented with a start and stop at the same time, as can many other data types.