Aggregating date field returns negative long value

[dai-chen]

Query with GROUP BY date field returns long value in which some are negative.

POST _opendistro/_sql
{
  "query": """
    SELECT
      timestamp, COUNT(*) FROM kibana_sample_data_flights GROUP BY timestamp LIMIT 5
  """
}

{
  "schema": [
    {
      "name": "timestamp",
      "type": "double"
    },
    {
      "name": "COUNT(*)",
      "type": "double"
    }
  ],
  "total": 5,
  "datarows": [
    [
      -1138807816,
      5
    ],
    [
      -1597045816,
      4
    ],
    [
      -1221134816,
      4
    ],
    [
      543486184,
      4
    ],
    [
      -1732792816,
      3
    ]
  ],
  "size": 5,
  "status": 200
}