title: Apache calcite: cast integer to datetime
body: <p>I am using Beam SQL and trying to cast integer to datetime field.</p>
  
  <pre class="lang-java prettyprint-override"><code>  Schema resultSchema =
      Schema.builder()
            .addInt64Field("detectedCount")
            .addStringField("sensor")
            .addInt64Field("timestamp")
            .build();
  
    PCollection&lt;Row&gt; sensorRawUnboundedTimestampedSubset = 
      sensorRowUnbounded.apply(
          SqlTransform.query(
            "select PCOLLECTION.payload.`value`.`count` detectedCount, \n"
            + "PCOLLECTION.payload.`value`.`id` sensor, \n"
            + "PCOLLECTION.`timestamp` `timestamp` \n"
            + "from PCOLLECTION "))
      .setRowSchema(resultSchema);
  </code></pre>
  
  <p>For some computation and windowing, I want to convert/cast <code>timestamp</code> to <code>Datetime</code> field? Please provide some pointers to convert <code>timestamp</code> in <code>resultSchema</code> to <code>DateTime</code>. datatype.</p>