how to access system.serializable objects in unity3d code example

Example: unity how to get data of play session time in a text file?

string path = Application.dataPath + "/Data/DataLog.txt";

        if (!File.Exists(path))
        {
            File.WriteAllText(path, "Level Test1 \n\n");
        }

        string date = "Login Date: " + System.DateTime.Now + "\n";
        string timed = "Session Time: " + System.Timers.Timer;

        File.AppendAllText(path, date);