Parsing extended JSON datetime

The datetime.json specification tests contains an example with the following json:

    "extjson": "{\"a\" : {\"$date\" : \"1960-12-24T12:15:30.500Z\"}}",

Converting this to bson with bson_new_from_json gives an error:

Could not parse "1960-12-24T12:15:30.500Z" as a date