Countdown for iPhone that continues while the apps is closed

System.DateTime.Now.ToUniversalTime(); this function returns the current time in a universal format. what you have to do is save it to local file (or playerprefs) and reload it back when you open the app again get the time using the same function. Now is you find the difference between these you will get the time offline, subtract it from the countdown time you have saved early to the local. The difference will be in System.TimeSpan so should know abt tht too

example:

this code i used to retrieve the difference in minutes

     string temp=PlayerPrefs.GetString("lastoffline");
     
     double lastMinutes = System.Double.Parse(temp);
     System.DateTime dt2 = System.DateTime.Now.ToUniversalTime();
     
     System.TimeSpan dt3 = dt2-((new System.DateTime(0))).ToUniversalTime();
     double currentMinutes = dt3.TotalMinutes;
     double totalOffline=currentMinutes-lastMinutes;

    // this code in start..

previously i have saved it from the OnApplicationQuit method of monobehaviour

     System.DateTime dt = System.DateTime.Now.ToUniversalTime();
     System.DateTime dt1 = new System.DateTime(0).ToUniversalTime();
     System.TimeSpan time1 = dt-dt1;
     double lastMinutes = time1.TotalMinutes;
 
     PlayerPrefs.SetString("lastoffline",(lastMinutes).ToString());

im changing double to string because playerprefs doesnt have option to save double,hence saving it as string and retrieving original data on loading