ctdNum.js

var seat = 43;
document.getElementById('countdown').innerHTML = seat;

const myInterval = setInterval(decre, 15000);

function showChange(){
  seat = seat - 2
  document.getElementById('countdown').innerHTML = seat;
}
function decre(){
    if(seat > 41){
      showChange()
    }else{
      clearInterval(myInterval);
      window.setTimeout(showChange, 60000);
    }
}