gurndsätzlich kann man die Options in Progam.cs angeben mit:

builder.Services.AddControllersWithViews()
    .AddNewtonsoftJson(options => { 
    options.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore;
    options.SerializerSettings.DateFormatString = "yyyy-MM-dd";
    options.SerializerSettings.NullValueHandling = NullValueHandling.Include;
    }
);

Dann in der IActionResult – Klasse des Controllers mit:

IActionResult() meineMethode(){

irgendeineKlasse myresult = Daten
.
.
.

 var settings = new JsonSerializerSettings
 {
     DateFormatString = "yyyy-MM-dd HH:mm:ss"
 };
 var json = JsonConvert.SerializeObject(myresult, settings);
 return Ok(json);
}