using NCrontab;
using System;
using System.Threading;

namespace ConsoleApp
{
    class Program // : BackgroundService
    {
        static private volatile Timer timer;
        static private volatile bool run;
        static private DateTime nextExecutionTime;

        static void Main(string[] args)
        {
            timer = new Timer((obj) =>
            {
                run = true;
                Console.WriteLine("Таймер сработал");
            }, null, TimeSpan.FromMilliseconds(-1), TimeSpan.FromMilliseconds(-1));

            ExecuteAsync();

            Console.ReadLine();
        }

        static public void ExecuteAsync() // protected override async Task ExecuteAsyn()
        {
            Console.WriteLine("Фоновый поток");

            while (true)
            {
                var shedule = CrontabSchedule.TryParse("*/1 * * * *"); // для отладки - будет срабатывать каждую минуту
                var start = DateTime.UtcNow;
                var next = shedule.GetNextOccurrence(start);
                if (next != nextExecutionTime)
                {
                    timer.Change(next - start, TimeSpan.FromMilliseconds(-1));
                    nextExecutionTime = next;
                    Console.WriteLine(next.ToShortTimeString());
                }

                if (run)
                {
                    Console.WriteLine("Запущено");
                    run = false;
                }

                Thread.Sleep(3000);  // await Task.Delay
            }
        }

    }
}