Once upon a time, somebody wrote a json parser that used an existing date parser because component reuse is the shizzle. Then somebody updated the date parser to handle more formats because they were using it in a different project. Congratulations, now you have a json parser that eats a multitude of date formats. (Some liberties with facts taken, but building a tower out of flexible components results in a flexible tower.)

When did that happen? There is a JSON standard, http://www.ietf.org/rfc/rfc4627.txt, but I have yet to find a parser that follows it, including Crockford's original "reference implementation". As a result, there's no point in developing a conformant parser, as no one would use it. They're off chasing date formats, capitalized keywords, functions (!!) blissfully unaware or willfully heedless of the impact on interoperability.