http://hl7.org/fhir/StructureDefinition/codesystem-effectiveDate|5.2.0

This is the first date-time when the value set version becomes active, so this value is present on Inactive value set versions as well. The start Date_time is expected to be as of 0001 UTC of the Effective Date.
NameFlagsCardTypeDescription & Constraints
Extension
0..1When the value set version becomes Active and is available for use
 
extension
?..0
 
url
?..?
 
value[x] *
1..?
 
 
valueDate *
1..?date