var serializerSettings = new JsonSerializerSettings
    {
        DateFormatHandling = DateFormatHandling.IsoDateFormat,
        DateTimeZoneHandling = DateTimeZoneHandling.Utc
    };

serializerSettings.Converters.Add(new IsoDateTimeConverter());

var jsonFormatter = new JsonMediaTypeFormatter { SerializerSettings = serializerSettings };
jsonFormatter.MediaTypeMappings.Add(GlobalConfiguration.Configuration.Formatters[0].MediaTypeMappings[0]);

GlobalConfiguration.Configuration.Formatters[0] = jsonFormatter;

WebApiConfig.Register(GlobalConfiguration.Configuration);