Can you share a sample JSON document that you are trying to query. I suspect something in the structure that is not aligned with the query.
I just did a quick test by creating two documents with a Date field and ran a query and it worked as expected
let prop1 = [
"title" : "doc_101"
let doc1 = Document(dictionary: prop1)
doc1.setDate(Date(), forKey: "date")
let prop2 = [
"title" : "doc_102"
let doc2 = Document(dictionary: prop2)
doc2.setDate(Date(), forKey: "date")
My query :
let searchQuery1 = Query.select(SelectResult.all())
sorts the results as expected!