title: display hours encompassing its ISO8601 offset in javascript
body: <p>I'm returned a set of ISO8601 dates, along with their offset.
  I would like to show the user the hour calculated according to that offset, not the local one. Unfortunately I don't have the timezone, in order to use the <code>Intl</code> interface.</p>
  <p>An example. Imagine I'm on a etc/GMT+4. If I'm returned this:</p>
  <pre><code>2019-05-30T07:00:00.000+01:00
  </code></pre>
  <p>I want to show the user the following:</p>
  <pre><code>07:00:00
  </code></pre>
  <p>not:</p>
  <pre><code>10:00
  </code></pre>