Check if this what you are looking for. I reduced the interval

If the modal is already popped you do not need to worry about the timer.

setInterval( () => {
	$("#myModal").modal();
}, 10000);

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.0.0/jquery.min.js"></script>

<!-- jQuery Modal -->
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery-modal/0.9.1/jquery.modal.min.js"></script>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/jquery-modal/0.9.1/jquery.modal.min.css" />

<div id="myModal" class="modal">
  <p>A Modal dialog</p>
  <a href="#" rel="modal:close">Close</a>
</div>

<!-- Link to open the modal -->
<p><a href="#myModal" rel="modal:open">Open Modal Manually</a></p>

Your code clears the interval if run for more than 30 seconds.

Remove the line with clearInterval, and it won’t stop:

var count=-1; 
var counter=setInterval(timer, 1000); 
function timer() 
{
  count=count+1;
  if (count >=30) 
  {
      $("#myModal").modal();
  }
    document.getElementById("timer").innerHTML=count + " secs"; // watch for spelling
}

I want my popup to appear every 30 seconds on my website.
and the pop will appear every 30 seconds onwards.
Now I only have javascript like this :

var count=-1; 
var counter=setInterval(timer, 1000); 
function timer()
{
  count=count+1;
  if (count >=30) 
  {
     clearInterval(counter);

      $("#myModal").modal();
     return;
  }
    document.getElementById("timer").innerHTML=count + " secs"; // watch for spelling
}

Thanks