Thanks to the joys of Twitter, it looks like this is just a matter of disabling the auto-DateTime-handling in Json.NET, e.g.
[...]JsonFormatter.SerializerSettings.DateParseHandling = DateParseHandling.None
It's possible that we should fix the configuration method to do this automatically...
Note that this will also disable the handling of "Date(...)"
values, so you do need to be careful with it. If you're in complete control of the caller of course, that shouldn't be a problem.