Why is my ISO datetime string with timezone converting the time upon POST?

Question

I am POSTing a JSON object as a string to a service method via the fetch API. Some of the properties in the object are ISO 8601 strings with timezones (eg "StartDate": "2019-04-16T13:46:04-06:00"). That is what the string looks like before POSTing via the fetch API. The C# REST service method that I am posting to has only ([FromBody]object document) as the parameter. Upon getting to that method, the string looks like this: "StartDate": "2019-04-16T19:46:04+00:00".

Why/where is the timezone getting converted? It was a string upon POSTing to the service and is still a string there.

Something to note: this works fine when using the service locally (via localhost). When it is deployed, it does not work.

Example code (TypeScript, client-side):

    postDocument() {
        let doc = "{'StartDate': '2019-04-16T13:46:04-06:00'}";
        let response = await fetch("[serviceURL]/api/Document/AddDocument", 
        {
            method: "POST",
            headers: {
              Accept: "application/json",
              "Content-Type": "application/json"
            },
            body: doc
        });
    }

Example code (server-side (in TestService), C#):

    [HttpPost]
    public void AddDocument([FromBody]object document)
    {
        // Datetime string has already been converted, no longer has timezone.
        console.log(document.ToString()); 
        // startdate here is = "2019-04-16T19:46:04+00:00"
        return;//Doesn't matter what's in this method
    }

Answer 1

Adding these lines to the Register method of my WebApiConfig in App_Start forced my service to not parse the DateTime and therefore it kept the timezone.

public static void Register(HttpConfiguration config)
{
    var jsonFormatter = config.Formatters.JsonFormatter;
    jsonFormatter.SerializerSettings.DateParseHandling = Newtonsoft.Json.DateParseHandling.None;
}