title: GraphQL.js - timestamp scalar type?
body: <p>I am building a GraphQL schema programmatically and in need of a <code>Timestamp</code> scalar type; a <a href="https://en.wikipedia.org/wiki/Unix_time" rel="noreferrer"><em>Unix Epoch timestamp</em></a> scalar type:</p>
  
  <pre class="lang-js prettyprint-override"><code>const TimelineType = new GraphQLObjectType({
    name: 'TimelineType',
    fields: () =&gt; ({
      date:  { type: new GraphQLNonNull(GraphQLTimestamp)  },
      price: { type: new GraphQLNonNull(GraphQLFloat)      },
      sold:  { type: new GraphQLNonNull(GraphQLInt)        }
    })
  });
  </code></pre>
  
  <p>Unfortunately, GraphQL.js <strong>doesn't</strong> have a <code>GraphQLTimestamp</code> nor a <code>GraphQLDate</code> type so the above doesn't work.</p>
  
  <p>I am expecting a <code>Date</code> input and I want to convert that to a timestamp. How would I go about creating my own GraphQL timestamp type?</p>