title: Add value to text box based on the value of another
body: <p>I just started coding and I am trying to make a little webpage that finds the date of birth of a user. So I would like to substract the age from the current year (2017). However I don't know how to do that. Can you help me?
  Html code:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;body&gt;
    &lt;h1 id="title"&gt; Date of birth finder
      &lt;h1&gt;
        &lt;h2 id="age"&gt; Insert your age here: &lt;/h2&gt;
        &lt;div&gt;
          &lt;form&gt;
            &lt;input type="number" name="quantity" min="0" max="2016"&gt;
          &lt;/form&gt;
        &lt;/div&gt;
        &lt;button id="bSubmit"&gt;Submit&lt;/button&gt;
        &lt;div class="user-answer"&gt;
          &lt;form&gt;
            &lt;input type="text" name="year"&gt;
          &lt;/form&gt;
        &lt;/div&gt;
  &lt;/body&gt;
  
  &lt;script&gt;
    var age = document.getElementById("quantity").value
    $("bSubmit").on(click, function {
  
    });
  &lt;/script&gt;</code></pre>
  </div>
  </div>
  </p>