Schedule time for add books C# SQL

I am working on a Library Management System and I want to add a feature that when students add a book there will be a return period of 7 days. I have tried something but it's not running as a background. Is it necessary to work in the background? And I have put a notification system in the student window.

What I have tried:

public class SchedulerService
{
    private static SchedulerService _instance;
    private List<Timer> timers = new List<Timer>();
        
    private SchedulerService() { }
        
    public static SchedulerService Instance => _instance ?? (_instance = new SchedulerService());
        
    public void ScheduleTask(int hour, int min, double intervalInHour, Action task)
    {
         DateTime now = DateTime.Now;
         DateTime firstRun = new DateTime(now.Year, now.Month, now.Day, hour, min, 0, 0);

         if (now > firstRun)
         {
             firstRun = firstRun.AddDays(1);
         }
        
         TimeSpan timeToGo = firstRun - now;

         if (timeToGo <= TimeSpan.Zero)
         {
             timeToGo = TimeSpan.Zero;
         }
        
         var timer = new Timer(x =>
                    {
                        task.Invoke();
                    }, null, timeToGo, TimeSpan.FromHours(intervalInHour));
        
         timers.Add(timer);
     }
}
    
public static class MyScheduler
{
    public static void IntervalInSeconds(int hour, int sec, double interval, Action task)
    {
        interval = interval / 3600;
        SchedulerService.Instance.ScheduleTask(hour, sec, interval, task);
    }

    public static void IntervalInMinutes(int hour, int min, double interval, Action task)
    {
        interval = interval / 60;
        SchedulerService.Instance.ScheduleTask(hour, min, interval, task);
    }

    public static void IntervalInHours(int hour, int min, double interval, Action task)
    {
        SchedulerService.Instance.ScheduleTask(hour, min, interval, task);
    }

    public static void IntervalInDays(int hour, int min, double interval, Action task)
    {
        interval = interval * 24;
        SchedulerService.Instance.ScheduleTask(hour, min, interval, task);
    }
}

private void Window_Loaded(object sender, RoutedEventArgs e)
        {
           
            Timer.MyScheduler.IntervalInDays(12, 00, 7,
            () => {
                this.Dispatcher.Invoke(() =>
                {
                    NotificatonTxtBox.Text = "You need to return your book" + Environment.NewLine;
                });
            });
        }