Subtract from selected time

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

mod = function(a, b) {
	return ((a%b)+b)%b;
}

myFunction = function() {
  var hour = parseInt(document.getElementById("selectHour").value); 
  var minute = parseInt(document.getElementById("selectMinute").value); 
  var ampm = document.getElementById("selectAMPM").value;
  
  // convert to 24 hour time
  if (ampm === "pm" && hour > 12) hour += 12;
  if (ampm === "am" && hour === 12) hour = 0; 
  
  var timeInHours = hour + (minute / 60);
  var timeToSubtractInHours  = 70 / 60;
  var timeDiff = (timeInHours - timeToSubtractInHours);
  var modDiff = mod(timeDiff,24);
  var h = modDiff | 0
  var min = (modDiff % 1) * 60 | 0;
  
  document.getElementById("result").innerHTML = h + ":" + min;
}