You could use this approach:

public class DateFormatConverter : IsoDateTimeConverter
{
    public DateFormatConverter(string format)
    {
        DateTimeFormat = format;
    }
}

And use it this way:

class ReturnObjectA 
{
    [JsonConverter(typeof(DateFormatConverter), "yyyy-MM-dd")]
    public DateTime ReturnDate { get;set;}
}

The DateTimeFormat string uses the .NET format string syntax described here: https://learn.microsoft.com/en-us/dotnet/standard/base-types/custom-date-and-time-format-strings

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);
}

Log.Debug oder Info oder Log.Error (System.Text.Json.JsonSerializer.Serialize(classVariable));