php - javascript function is not working on button click. But working without button click

In this function, I have implemented a timer which is working fine in the given html input box without onclick function 'start_test'. but i want to start this inner function on button click which is not working. PLease help me to find the mistake.

function start_test() {

        var hms = "01:30:00";
        var a = hms.split(':'); // split it at the colons
        
        // minutes are worth 60 seconds. Hours are worth 60 minutes.
        var seconds = (+a[0]) * 60 * 60 + (+a[1]) * 60 + (+a[2]); 
        
        if( seconds > 0 ){  
          function secondPassed() {
              var minutes = Math.round((seconds - 30)/60),
                  remainingSeconds = seconds % 60;

              if (remainingSeconds < 10) {
                  remainingSeconds = "0" + remainingSeconds;
              }

              document.getElementById('countdown').innerHTML = " " +minutes + ":" + remainingSeconds;
              if (seconds == 0) {
                  clearInterval(countdownTimer);
                 //form1 is your form name
                document.form_quiz.submit();
              } else {
                  seconds--;
              }
          }
          var countdownTimer = setInterval('secondPassed()', 1000);

          } 
    }
<div class="col-md-4" style="text-align: right;">
    <button  class="btn btn-primary" id="start_test" onclick="start_test();" >Start Test</button>
        <input type="hidden" value="<?php echo date('M d Y');?>" id="c_month">
        <h2><time id="countdown">01:30:00</time>   </h2>          
</div>
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