Example request and (correct) response when using a script terms facet in elasticsearch 0.20.6 (+ lang-javascript 1.2.0).

gistfile1.js

{
  "query" : {
    "match_all" : { }
  },
  "facets" : {
    "lt322cf7ts" : {
      "terms" : {
        "size" : 31,
        "script" : "t=doc['timestamp'].date;t.addMillis(-28800000);v=t.getHourOfDay();v",
        "lang" : "js"
      }
    }
  }
}
--
{
  "took" : 261,
  "timed_out" : false,
  "_shards" : {
    "total" : 1,
    "successful" : 1,
    "failed" : 0
  },
  "hits" : {
    "total" : 3,
    "max_score" : 1.0,
    "hits" : [ {
      "_index" : "4oerq1pia8",
      "_type" : "event",
      "_id" : "ci7842krt3",
      "_score" : 1.0, "_source" : {"timestamp":"2012-11-30T15:00:00.000-08:00","@id":"ci7842krt3"}
    }, {
      "_index" : "4oerq1pia8",
      "_type" : "event",
      "_id" : "ls41ppd5us",
      "_score" : 1.0, "_source" : {"timestamp":"2012-12-11T15:00:00.000Z","@id":"ls41ppd5us"}
    }, {
      "_index" : "4oerq1pia8",
      "_type" : "event",
      "_id" : "plc3s8h4rq",
      "_score" : 1.0, "_source" : {"timestamp":"2012-12-02T15:00:00.000-08:00","@id":"plc3s8h4rq"}
    } ]
  },
  "facets" : {
    "lt322cf7ts" : {
      "_type" : "terms",
      "missing" : 0,
      "total" : 3,
      "other" : 0,
      "terms" : [ {
        "term" : "15",
        "count" : 2
      }, {
        "term" : "7",
        "count" : 1
      } ]
    }
  }
}