ScheduledTaskManager.cs

// Contract
public interface IScheduledTaskManager
{
    void AddAbsoluteTask(Action task, int absoluteMinute);
    void AddAbsoluteTask(Action task, int absoluteHour, int absoluteMinute);
    void AddSlidingTask(Action task, int slidingMinute);
    void AddSlidingTask(Action task, int slidingHour, int slidingMinute);
}

// Main Class
public class ScheduledTaskManager
    : IScheduledTaskManager , IDisposable
{
    List<Task> Tasks = new List<Task>();
    private Timer Timer;
    private Func<DateTime> Watch;

    public ScheduledTaskManager(Func<DateTime> currentTimeWatch = null)
    {
        Watch = currentTimeWatch ?? new Func<DateTime>(Helper.GetCurrentDatetime);
        Timer = new System.Timers.Timer(1000);
        Timer.Elapsed += Timer_Elapsed;
        Timer.Start();
    }
       
    void Timer_Elapsed(object sender, ElapsedEventArgs e)
    {
        foreach (var t in Tasks)
            if (t.IsTime())
                t.Action.Invoke();
    }

    public void Dispose()
    {
        Timer.Stop();
        Timer.Dispose();
    }
       
    /// Run at every hour at given minute
    public void AddAbsoluteTask(Action task, int absoluteMinute)
    {
        Tasks.Add(new Task { Action = task, AbsoluteMinute = absoluteMinute });
    }

    /// Run every day at given hour and minute
    public void AddAbsoluteTask(Action task, int absoluteHour, int absoluteMinute)
    {
        Tasks.Add(new Task { Action = task, AbsoluteMinute = absoluteMinute, AbsoluteHour = absoluteHour });
    }

    /// Run after given minutes from last execution
    public void AddSlidingTask(Action task, int slidingMinute)
    {
        Tasks.Add(new Task { Action = task, SlidingMinute = slidingMinute });
    }

    /// Run after given hours + minutes from last execution
    public void AddSlidingTask(Action task, int slidingHour, int slidingMinute)
    {
        Tasks.Add(new Task { Action = task, SlidingMinute = slidingMinute, SlidingHour = slidingHour });
    }

}