In context of https://jira.hl7.org/browse/FHIR-28375 we propose that the Data Type list be limited to dateTime while also indicating that Period is an optional choice.

Laboratory observations are measurements and simple assertions which, by nature, are at a point in time and not periods.  If there is interest in a period, e.g., how long a culture was incubated before measuring, that would not be captured in .effective.  While the description states that this reflects the specimen collection date/time, as that is for lab tests the physiologically relevant date/time, if specimen collection took some time then that period would be the specimen collection date time in Specimen.collection.collected[x] where a Period is available.  Observation.effective would use a reasonable point in time reflecting that collection.  Note that also in HL7 v2, which typically would transmit the source data to the EHR, there is no support for Period for an effective date/time of the observation (OBX-14). 

As the FHIR US Core profile is used for accessing and exchanging data based on what the source system has, this should not require the data source system to add functionality to enable Period as part of Observation.effective[x]. Additionally, C-CDA does not impose this requirement either.

Links to pages:

http://hl7.org/fhir/us/core/STU3.1/StructureDefinition-us-core-observation-lab.html

https://build.fhir.org/ig/HL7/US-Core-R4/StructureDefinition-us-core-observation-lab.html