When an ASP.NET MVC 4 action returns JSON containing a DateTime value, the value is serialized into a format that cannot be natively understood by JavaScript. Here is some code to process the value into a JavaScript Date.
A better solution than this may be to replace the built-in JsonValueProviderFactory with a JSON.NET-based alternative (JsonDotNetValueProviderFactory).
See http://www.hanselman.com/blog/OnTheNightmareThatIsJSONDatesPlusJSONNETAndASPNETWebAPI.aspx and http://www.dalsoft.co.uk/blog/index.php/2012/01/10/asp-net-mvc-3-improved-jsonvalueproviderfactory-using-json-net and https://gist.github.com/DalSoft/1588818 and http://stackoverflow.com/questions/11490506/mvc3-controller-with-specific-jsonconverter