private function getTimestamp($key)
{
// Select the timestamp from the table
$statement = new SimpleStatement($this->selectQuery);
$options = new ExecutionOptions(array("arguments" => array("key" => $key)));
$rows = $this->session->execute($statement, $options);
$row = $rows->first();
$this->assertArrayHasKey("writetime(value_int)", $row);
// Return the timestamp
return current($row)->value();
}