title: Google App Engine Cron schedule and length
body: <p>What is the min and max of cron schedule? And how long the cron can run? 24 hours?
  The below code work with schedule is 1 minutes. </p>
  
  <pre><code>   cron:
      - description: test
        url: /test
        schedule: every 1 minutes
  </code></pre>
  
  <p>But I want to make it 30 seconds</p>
  
  <p>So I wrote</p>
  
  <pre><code> cron:
      - description: test
        url: /test
        schedule: every 30 seconds
  </code></pre>
  
  <p>But it just return error when I update the app.</p>