1 min countdown timer does not work in inactive tab.it keeps starting at 59:59

Question

I made a one minute countdown timer in JavaScript. it works properly and announces result when I am on The timer tab. The problem is when the timer starts and i switch to another tab and come back to the timer tab, its counting time correctly but once it reaches 00:00 it starts counting as 59:58. it does not announce result. what's wrong with the code?

var myVar = setInterval(myTimer, 1000);

function myTimer() {
  var xmlhttp = new XMLHttpRequest();
  xmlhttp.open("GET", "free_dance_response.php", false);
  xmlhttp.send(null);
  if (xmlhttp.responseText == "Time Out") {
    myStopFunction();
    //quitWarning_timer();
  }
  $('#response1').html(xmlhttp.responseText);
}

function myStopFunction() {
  clearInterval(myVar);
}