Save current time

     using UnityEngine;
     using System.Collections;
     using System;

     public class DataMaster : MonoBehaviour
     {
         DateTime currentDate;
         DateTime oldDate;
         void Start()
         {
             //Store the current time when it starts
             currentDate = System.DateTime.Now;

             //Grab the old time from the player prefs as a long
             long temp = Convert.ToInt64(PlayerPrefs.GetString("sysString"));

             //Convert the old time from binary to a DataTime variable
             DateTime oldDate = DateTime.FromBinary(temp);
             print("oldDate: " + oldDate);

             //Use the Subtract method and store the result as a timespan variable
             TimeSpan difference = currentDate.Subtract(oldDate);
             print("Difference: " + difference);

         }

         void OnApplicationQuit()
         {
             //Savee the current system time as a string in the player prefs class
             PlayerPrefs.SetString("sysString", System.DateTime.Now.ToBinary().ToString());

             print("Saving this date to prefs: " + System.DateTime.Now);
         }

     }