US Core design pattern is to mark min=1 when the data element is always expected to be known by every system. However, based on vendor feedback, we are not confident this is the case for the Observation.effectivePeriod. The US Core Must Support definition states: US Core Responders  SHALL  be capable of populating all data elements as part of the query results as specified by the  US Core Server Capability Statement . ... When querying US Core Responders, US Core Requestors  SHALL  interpret missing data elements within resource instances as data not present in the US Core Responder’s system. Therefore, min=0 Must Support means that if the data is present, the element is populated.