title: GraphQL query filter by current timestamp
body: <p>I'm wondering what would be a nice solution to <strong>filter GraphQL queries by the current timestamp</strong>, e.g:</p>
  
  <pre><code>query {
    appointments(createdAt: today) { }
  }
  </code></pre>
  
  <p>or</p>
  
  <pre><code>query {
    appointments(createdAt_gte: now) { }
  }
  </code></pre>
  
  <p>Is it recommended to create my own resolver functions here? How would they look like, when e.g. graph.cool is used as a backend?</p>
  
  <p>I don't want to rely on the correct time of the client but use the server time instead.</p>