Difference of times

Hi, I want to do a script that will calculate the difference: present time and past time, who write this before closing the application and changes it to seconds. But I wrote a script that does not work very well.

       public class TIMESAVE : MonoBehaviour {
       DateTime NowTime;
       DateTime OldTime;
 public UnityEngine.UI.Text now;
 void Start()
 {
     DateTime NowTime = System.DateTime.Now;

     long temp = Convert.ToInt64(PlayerPrefs.GetString("wtf"));

     DateTime oldTime = DateTime.FromBinary(temp);

     TimeSpan difference = NowTime.Subtract(OldTime);
     now.text = "roznica wynosi: " + difference.Minutes + ":" + difference.Seconds + "\n Aktualny: " + NowTime.Hour + ":" + NowTime.Minute + ":" + NowTime.Second + "\n Ostatni: " + OldTime.Hour + ":" + OldTime.Minute + ":" + OldTime.Second;
 }
 void OnApplicationQuit()
 {
     PlayerPrefs.SetString("wtf", System.DateTime.Now.ToBinary().ToString());
 }

}