JSON JObject.Parse modifies json string

It does not really modify your string, it just parses your date string into DateTime object when you call JObject.Parse. If you do this:

var obj = JObject.Parse(json);var values = (JArray) obj["valuesList"];var time = (JValue) values[0]["TimeCaptured"];Console.WriteLine(time.Value.GetType());

You notice that time.Value is of type DateTime. Then you do this:

JsonConvert.DeserializeObject<List<IValuePacket>>(jobject["valuesList"].ToString());

By doing that you convert valueList back to json, but now TimeCaptured is DateTime and not a string, so that DateTime object is converted to json string using whatever date time format is used by JSON.NET by default.

You can avoid parsing strings that look like dates to .NET DateTime objects by parsing json to JObject like this:

JObject obj;using (var reader = new JsonTextReader(new StringReader(json))) {    // DateParseHandling.None is what you need    reader.DateParseHandling = DateParseHandling.None;    obj = JObject.Load(reader);}

Then type of TimeCaptured will be string, as you expect.

Side note: there is no need to convert JToken back to string and then call JsonConvert.Deserialize on that string. Instead, do this:

var values = obj["valuesList"].ToObject<List<IValuePacket>>();