> There's no way to tell staring at a JSON string if "2018-04-22" is meant to be a date rather than a text string.

I believe the op meant you should do something like this:

"created": { "type": "datetime", "format": "iso8601", "value": "2018-04-etc" }

Now there's no ambiguity and the serialization is still json compliant. You have to let go of the notion that you can just put a date formatted string in there and things will magically work.