NextRun does sometimes not get updated or updated incorrectly

[ExtraTNT]

So, it looks like that the NextRun value does sometimes not get updated or does get updated incorrectly.
Which leads to tasks being executed twice when they should instead only run once.
I wrote a small test application to show the problem:

...
private async Task<bool> Tester()
    {
        _logger.LogInformation("Thread id: " + Thread.CurrentThread.ManagedThreadId);
        _logger.LogInformation("Next Run: " + _scheduler.NextRun);
        _logger.LogInformation("Current Time " + DateTime.Now);
        // do something for 1 sec... (sleep 2x 1/2 sec -.-)
        Thread.Sleep(500);
        Thread.Sleep(500);
        _logger.LogInformation("After execution Time (+ 1s): " + DateTime.Now);
        return true;
    }

I ran this every 10 seconds and got the following output:

testScheduler.Worker: Information: Thread id: 9
testScheduler.Worker: Information: Next Run: 04.04.2022 14:43:40
testScheduler.Worker: Information: Current Time 04.04.2022 14:43:30
testScheduler.Worker: Information: After execution Time (+ 1s): 04.04.2022 14:43:31
Exited Thread 302116
testScheduler.Worker: Information: Thread id: 9
testScheduler.Worker: Information: Next Run: 04.04.2022 14:43:50
testScheduler.Worker: Information: Current Time 04.04.2022 14:43:40
testScheduler.Worker: Information: After execution Time (+ 1s): 04.04.2022 14:43:41
Exited Thread 302587
Started Thread 303062
testScheduler.Worker: Information: Thread id: 3
testScheduler.Worker: Information: Next Run: 04.04.2022 14:44:00
testScheduler.Worker: Information: Current Time 04.04.2022 14:43:50
testScheduler.Worker: Information: After execution Time (+ 1s): 04.04.2022 14:43:51
Exited Thread 303063
testScheduler.Worker: Information: Thread id: 7
testScheduler.Worker: Information: Next Run: 04.04.2022 14:44:00
testScheduler.Worker: Information: Current Time 04.04.2022 14:44:00
testScheduler.Worker: Information: After execution Time (+ 1s): 04.04.2022 14:44:01
testScheduler.Worker: Information: Thread id: 3
testScheduler.Worker: Information: Next Run: 04.04.2022 14:44:10
testScheduler.Worker: Information: Current Time 04.04.2022 14:44:01
testScheduler.Worker: Information: After execution Time (+ 1s): 04.04.2022 14:44:02
Exited Thread 303539
testScheduler.Worker: Information: Thread id: 7
testScheduler.Worker: Information: Next Run: 04.04.2022 14:44:10
testScheduler.Worker: Information: Current Time 04.04.2022 14:44:10
testScheduler.Worker: Information: After execution Time (+ 1s): 04.04.2022 14:44:11
testScheduler.Worker: Information: Thread id: 3
testScheduler.Worker: Information: Next Run: 04.04.2022 14:44:20
testScheduler.Worker: Information: Current Time 04.04.2022 14:44:11
testScheduler.Worker: Information: After execution Time (+ 1s): 04.04.2022 14:44:12

The bug occurs randomly, but in a test before a "buggy thread" was more likely to bug again and it got worse over time. But since this is not really testable i can not confirm, that it will always be this way.
I currently do not have a fix for this bug, but i will try to fix it and create a pull request.
The version i use is version-6 with dotnet 6.0 on debian gnu/linux 11

[ExtraTNT]

It looks like sometimes RunNext gets calculated before the task should run, leading to a wrong RunNext time, without an error in the calculation...

[ExtraTNT]

found the problem, it's in InternalSchedule in Run, using Calculator.Now(); in the calculation for the delay isn't accurate, because a cpu takes time to do something...

i have a fix for it, but it probably will not work if the task runs every second (gets executed ~1 sec later -> probably, because we don't have infinitely fast cpus). i will test this and if it's a problem fix it

[ExtraTNT]

Found a easy way to fix the 1 sec issue, a bit hacky, but should work