Does anyone know why I get the datetime?

You handle it in JavaScript

When you receive the data in your ajax call, you deserialize it in this way:

var data = JSON.parse(data, function(key, value) { 
    if (typeof value === 'string') { 
        var d = /\/Date\((\d*)\)\//.exec(value); 
        return (d) ? new Date(+d[1]) : value; 
    } 
    return value; 
});

the function parse optionally receives a function as the second parameter that allows you to control how to interpret the data. In this example, you use a regex to identify the strings that are date formatted.

You format it in the controller before serializing

Instead of sending properties DateTime , you create another object that has a string with the date in the format you want. You have not shown us the code of the action that sends the data, if you added it, it would be easier to indicate how to do it.

You use an alternate serializer

With a little configuration you can Json.NET library that is more flexible and allows you to choose how the dates will be serialized, in Microsoft format ( /Date(1542333807388)/ ) or ISO ( 2018-11-16T02:03:27.388Z ).

You create a class derived from JsonResult :

public class CustomJsonResult : JsonResult
{
    private const string _dateFormat = "yyyy-MM-dd HH:mm:ss";

    public override void ExecuteResult(ControllerContext context)
    {
        if (context == null)
        {
            throw new ArgumentNullException("context");
        }

        HttpResponseBase response = context.HttpContext.Response;

        if (!String.IsNullOrEmpty(ContentType))
        {
            response.ContentType = ContentType;
        }
        else
        {
            response.ContentType = "application/json";
        }
        if (ContentEncoding != null)
        {
            response.ContentEncoding = ContentEncoding;
        }
        if (Data != null)
        {
            // Using Json.NET serializer
            var isoConvert = new IsoDateTimeConverter();
            isoConvert.DateTimeFormat = _dateFormat;
            response.Write(JsonConvert.SerializeObject(Data, isoConvert));
        }
    }
}

and then in the controller you use it like this:

[HttpGet]
public ActionResult Index() {
    return new CustomJsonResult { Data = new { fecha = DateTime.Now } };
}