TimeOnlyConverter.cs

public class TimeOnlyConverter : JsonConverter<TimeOnly>
{
	private readonly string serializationFormat;

	public TimeOnlyConverter() : this(null) { }

	public TimeOnlyConverter(string? serializationFormat)
	{
		this.serializationFormat = serializationFormat ?? "HH:mm:ss.fff";
	}

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

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