You can use a Timer to check each minute if DateTime.Now == (the specific time you want)
This is an example of a code with windows forms
public MainWindow()
{
InitializeComponent();
System.Windows.Threading.DispatcherTimer timer_1 = new System.Windows.Threading.DispatcherTimer();
timer_1.Interval = new TimeSpan(0, 1, 0);
timer_1.Tick += new EventHandler(timer_1_Tick);
Form1 alert = new Form1();
}
List alarms = new List();
public struct Alarm
{
public DateTime alarm_time;
public string message;
}
public void timer_1_Tick(object sender, EventArgs e)
{
foreach (Alarm i in alarms) if (DateTime.Now > i.alarm_time) { Form1.Show(); Form1.label1.Text = i.message; }
}