I'll agree there aren't many ways this could be solved without breaking compatibility with JSON. Certainly something like {"created": Date("2013-12-1T12:00:00Z")} a la BSON seems elegant but is incompatible with JSON.

Maybe something like "String values that match (some computationally inexpensive ISO-8601-matching regex) shall be converted to Date instances by the JSON parser" could be possible without huge compatibility issues.

I guess I was mostly using this topic to voice what I'd imagine is a common point of frustration in an otherwise great data interchange format.