title: Convert yyyyMMdd string into datetime in Presto
body: <p>I have a birthdate column with date in form of '20100731', I want to convert it into datetime therefore I can get its user's age.</p>
  <p>I've tried multiple casting however no luck.</p>
  <p>I can only think of doing</p>
  <pre><code>year(current_date) - cast(substr('20100731', 1,4) as int)  
  </code></pre>
  <p>This works fine but I want to know way to convert 'yyyyMMdd' string into datetime format</p>