Display text when timer reaches 0s, and add 0 before 1-9s

I want to display some text once the timer goes to 0:00, and when the timer is anywhere in between 1-9s, I want to display a 0 in front of the seconds, e.g. 1:04 instead of 1: 4. How would I do this?

const timeSpan = document.getElementById('timer');
var sec = 600 / 60;
const mins = sec;
const now = new Date().getTime();
const deadline = mins * 60 * 1000 + now;


setInterval(() => {
  var currentTime = new Date().getTime();
  var distance = deadline - currentTime;
  var minutes = Math.floor((distance % (1000 * 60 * 60)) / (1000 * 60));
  var seconds = Math.floor((distance % (1000 * 60)) / 1000);


  timeSpan.innerHTML = minutes + ':' + seconds;
  
}, 500)