Javascript timer stops if browser tab is minimized

I have several storyline projects that use a javascript countdown timer so that certain activities are completed in a certain time.

I've noticed that in some browsers, the timer stops if you minimize a tab, in others, the countdown continues. I want it always to continue. I believe it has something to do with browser throttling???

Any ideas how I could make sure the timer never stops, even if the tab/window is minimized?

Here's the existing code (made using Matthew Bibby's excellent but sadly defunct website):

function zeros(i) {
if (i < 10) {
    i = "0" + i;
}
return i;
}
var minutes, seconds, timer, totalTime;
var counter = setInterval(timer, 1000);
var fin="Done";
var player = GetPlayer();
var count = player.GetVar("Countdown_Duration");
function timer() {
    count = count - 1;
    minutes = zeros(Math.floor(count / 60));
    seconds = zeros(count - minutes * 60);
    if (count == 0){
        player.SetVar("Countdown_Finished",fin);
}
    if (count < 0) {
        clearInterval(counter);
        return;
    }
    totalTime = minutes + ':' + seconds;
    player.SetVar("Countdown_Display",totalTime);
}

Thank you for any help!

 

C.