Hide Div onClick

HTML

<a href="#" onclick="show('mydiv')">Open DIV</a>
<div id="mydiv">
    <div id="mydiv-container">
        <div id="mydiv-content">
        <h1>Current Time Left</h1>
        <div id="countdown"></div>
        <br>Click the link to close.
        <br>
        <a href="#" onclick="hide('mydiv')">Close</a>
        </div>
    </div>
</div>

CSS

body {
    height: 100%;
    background-color: #F0F0F0;
    font-family: Arial, sans-serif;
}
#mydiv {
    width: 100%;
    height: 100%;
    overflow: hidden;
    left: 100px;
    top: 100px;
    position: absolute;
    opacity: 0.5;
    z-index: 200;
}
#mydiv-container {
    margin-left: auto;
    margin-right: auto;
}
#mydiv-content {
    width: 70%;
    padding: 20px;
    background-color: white;
    border: 1px solid #6089F7;
}
a {
    color: #5874BF;
    text-decoration: none;
}
a:hover {
    color: #112763;
}

JavaScript

function show(target) {
    document.getElementById(target).style.display = 'block';
}

function hide(target) {
    document.getElementById(target).style.display = 'none';
}

function countdown(elementName, minutes, seconds) {
  var element, endTime, hours, mins, msLeft, time;

  function twoDigits(n) {
    return (n <= 9 ? "0" + n : n);
  }

  function updateTimer() {
    msLeft = endTime - (+new Date);
    if (msLeft < 1000) {
      element.innerHTML = "countdown's over!";
    } else {
      time = new Date(msLeft);
      hours = time.getUTCHours();
      mins = time.getUTCMinutes();
      element.innerHTML = (hours ? hours + ':' + twoDigits(mins) : mins) + ':' + twoDigits(time.getUTCSeconds());
      setTimeout(updateTimer, time.getUTCMilliseconds() + 500);
    }
  }

  element = document.getElementById(elementName);
  endTime = (+new Date) + 1000 * (60 * minutes + seconds) + 500;
  updateTimer();
}

countdown("countdown", 1, 00);