https://github.com/w3c/activitystreams/issues/542

as:closed is defined as Object | Link | xsd:boolean | xsd:datetime

the normative context explicitly types the value to a datetime

solution: define closedBy as a term definition for as:closed but with @type: @id

1  "as": "https://www.w3.org/ns/activitystreams#",
2  // ...
3  "closedBy": {
4    "@id": "as:closed",
5	 "@type": "@id"
6  },
7  // ...