eHealth Infrastructure (v2020.1) 

{
  "resourceType" : "SearchParameter",
  "id" : "ehealth-communication-search-period",
  "text" : {
    "status" : "generated",
    "div" : "<div xmlns=\"http://www.w3.org/1999/xhtml\"><p><b>Generated Narrative with Details</b></p><p><b>id</b>: ehealth-communication-search-period</p><p><b>url</b>: <a href=\"http://ehealth.sundhed.dk/fhir/SearchParameter/Communication/period\">http://ehealth.sundhed.dk/fhir/SearchParameter/Communication/period</a></p><p><b>name</b>: period</p><p><b>status</b>: active</p><p><b>date</b>: Feb 4, 2020 9:11:17 AM</p><p><b>code</b>: period</p><p><b>base</b>: Communication</p><p><b>type</b>: date</p><p><b>description</b>: Search parameter for finding communication based on period</p><p><b>expression</b>: Communication.extension('http://ehealth.sundhed.dk/fhir/StructureDefinition/ehealth-period')</p><p><b>xpathUsage</b>: normal</p></div>"
  },
  "url" : "http://ehealth.sundhed.dk/fhir/SearchParameter/Communication/period",
  "name" : "period",
  "status" : "active",
  "date" : "2020-02-04T09:11:17+00:00",
  "code" : "period",
  "base" : [
    "Communication"
  ],
  "type" : "date",
  "description" : "Search parameter for finding communication based on period",
  "expression" : "Communication.extension('http://ehealth.sundhed.dk/fhir/StructureDefinition/ehealth-period')",
  "xpathUsage" : "normal"
}