Subtract from selected time with moment.js

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.22.1/moment.min.js"></script>
<select id="selectHour">
    <option value="1">1</option>
    <option value="2">2</option>
    <option value="3">3</option>
    <option value="4">4</option>
    <option value="5">5</option>
    <option value="6">6</option>
    <option value="7">7</option>
    <option value="8">8</option>
    <option value="9">9</option>
    <option value="10">10</option>
    <option value="11">11</option>
    <option value="12">12</option>
</select>

<select id="selectMinute">
    <option value="00">00</option>
    <option value="5">5</option>
    <option value="10">10</option>
    <option value="15">15</option>
    <option value="20">20</option>
    <option value="25">25</option>
    <option value="30">30</option>
    <option value="35">35</option>
    <option value="40">40</option>
    <option value="45">45</option>
    <option value="50">50</option>
    <option value="55">55</option>
</select>

<select id="selectAMPM">
    <option value="am">am</option>
    <option value="pm">pm</option>
</select>

<button onclick="myFunction()">Get Result</button>
<p id="result"></p>

JavaScript

// https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.22.1/moment.min.js

myFunction = function() {
  var hour = document.getElementById("selectHour").value; 
  var minute = document.getElementById("selectMinute").value; 
  var ampm = document.getElementById("selectAMPM").value;
  var time = hour + ":" + minute + " " + ampm;
  var m = new moment(time, 'h:m a');
  
  
  document.getElementById("result").innerHTML = m.subtract(70, 'minutes').format('h:m a'); 
}