Countdown timer not working when added form attribute

HTML-CSS
1

Hey guys!

I have a countdown timer and there are several issues here. Whenever, I click on the start button, the timer does start but if I keep clicking on it, it will speed it up. I also can’t get the buttons to align it to the center when I did margin 15% auto for my main div class and when I tried to include the form attribute, the timer stops working for some reason but works without the form attribute:

<?php

$timer = 60;


?>





<!DOCTYPE html>
<html>
<head>
	<title></title>
	<link rel="stylesheet" type="text/css" href="style.css">
	<link href="https://fonts.googleapis.com/css?family=Lobster" rel="stylesheet">
</head>
<body>


<script>
   var timer = <?php echo $timer ?>;    // 1 minute timer.... 
   var min = 0;
   var sec = 0;

   function startTimer() {
   	   min = parseInt(timer/60);
   	   sec = parseInt(timer%60);

   	   if(timer<1) {
   	   	   document.write("You have to start again!");
   	   }

   	   document.getElementById("time").innerHTML = "<b>Time Left:  </b>" + min.toString() + ":" + sec.toString();
   	   timer--;
   	   setTimeout(function() {
          startTimer();
   	   }, 1000);
   }



     function stop() {
     	
     	alert("Thank you for completing the test. You finished the test at: " +  min.toString() + ":" + sec.toString());
     	 
         var result = document.getElementById('time').innerHTML;
         document.getElementById('input').value = result;
    //   window.location.href = "update.php";
     }



</script>
<div class="timer">
<h1>Welcome to Timertrone.</h1>
<br></br>


   <div class="timer_display"><center><b><span id="time" ...></span></center></b></div>


<form>
<input type="text" id="input">
<h1 id="buttons"></h1>
<button type="submit" name="start" onclick="startTimer()">Start</button>
<button type="submit" name="start" onclick="stop()">Stop</button>
</div>
</form>
</body>
</html>


Here is my CSS code:

div.timer {
  margin: 15% auto;

}

 #buttons {

position: relative;
left: 200px;
}

div.timer_display {
   font-family: 'Lobster', cursive;
   font-size: 40px;
   color: blue;
   padding-top: 100px;
}