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