start_duration_agg.sh

DELETE test 

PUT test
{
  "mappings": {
    "test": {
      "properties": {
        "start": {
          "type": "date"
        },
        "duration": {
          "type": "long"
        }
      }
    }
  }
}

PUT test/test/1
{
  "start": "2015-07-12T09:14:01Z",
  "duration": 10000
}

PUT test/test/2
{
  "start": "2015-07-12T10:20:50Z",
  "duration": 20000
}

PUT test/test/3
{
  "start": "2015-07-12T12:00:42Z",
  "duration": 5000
}

GET test/_search
{
  "aggs": {
    "my_histo": {
      "date_histogram": {
        "script": "start = doc['start'].value; duration = doc['duration'].value * 1000; l = []; for (long i = 0; i < duration; i += interval) { l.add(start + i); }; return l;",
        "params": {
          "interval": 3600
        },
        "interval": "hour"
      }
    }
  }
}

I have a question about your script. You return "l" to elasticsearch. How does elasticsearch use that value?