What you can do (well, assuming you have enough control over the JSON), is omit the date field for documents which have no date. For example these two documents will work together in a single Discovery collection.
{
"title": "Document With Date",
"text": "Discovery detects date types to support range queries, sorting and more.",
"updated": "2018-04-26T10:11:12Z"
}
{
"title": "Undated Document",
"text": "Discovery has no trouble with fields that appear in some documents and not others."
}
