DateOnlyConverter.cs

public class DateOnlyConverter : JsonConverter<DateOnly>
{
    private readonly string serializationFormat;
    public DateOnlyConverter() : this(null) { }

    public DateOnlyConverter(string? serializationFormat)
    {
        this.serializationFormat = serializationFormat ?? "yyyy-MM-dd";
    }

    public override DateOnly Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        var value = reader.GetString();
        return DateOnly.Parse(value!);
    }

    public override void Write(Utf8JsonWriter writer, DateOnly value, JsonSerializerOptions options)
        => writer.WriteStringValue(value.ToString(serializationFormat));
}