Timer drags a bit when too much is happening - flutter

Question

I'm working on a timer that needs to do some calculations and run some functions at a certain interval. I'm trying to keep the interval as big as possible, but I need it to be kind of fine grained.
Here is the periodic timer with some of the stuff that needs to happen.
So as you can see, every second (the milliseconds passed % 1000 == 0) it will do some stuff if some conditions are met. But also every 10 milliseconds I need to check some stuff.
It seems this is a bit too much, and after running the timer for 2 minutes it already drags 1 second behind. I guess I'm approaching this the wrong way. Could/should I somehow put all that logic in a function that just runs async so the timer can just keep going.
It's not the end of the world if the timer display drags for a few milliseconds every now and then, if it catches up later. But now the whole timer just drags.
_timer = Timer.periodic(Duration(milliseconds: 10), (timer) {
passedMilliseconds = passedMilliseconds + 10;
// METRONOME BEEP AND BLINK
if (passedMilliseconds % currentTimerSettingsObject.milliSecondDivider == 0) {
_playMetronomeBeep();
_startMetronomeBlink();
}
// ONE SECOND
if (passedMilliseconds % 1000 == 0) {
secondsDuration--;
// COUNTDOWN
if (secondsDuration < currentTimerSettingsObject.countDown + 1) {
_player.play('sounds/beep.mp3');
}
// SET COUNTDOWN START VALUES
if (secondsDuration == currentTimerSettingsObject.countDown) {
isCountdown = true;
}
notifyListeners();
}
// TIME IS UP
if (secondsDuration < 0) {
switchToNextTimer();
notifyListeners();
}
});
}

Answer 1

You cannot rely on a timer to deliver events exactly on time. You need to use a more exact method than simply incrementing a counter by 10 on every tick. One example would be to start a Stopwatch before the timer and then (knowing that your ticks will only be on approximately 10ms intervals) read stopwatch.elapsedMilliseconds and base your decisions on that.
You will need to change your logic a bit. For example, you want to know when you pass a 1 second boundary. Previously, with your exact increments of 10 you knew you would eventually reach a round 1000. Now, you might see 995 followed by 1006, and need to deduce that you've crossed a second boundary to run your per second logic.