Sub aggregation and last document retrieval based on field

Elastic Stack Elasticsearch
1 
{
    "took": 53,
    "timed_out": false,
    "_shards": {
        "total": 2,
        "successful": 2,
        "failed": 0
    },
    "hits": {
        "total": 6,
        "max_score": 1.0,
        "hits": [{
            "_index": "db",
            "_type": "users",
            "_id": "AVOiyjHmzUObmc5euUGS",
            "_score": 1.0,
            "_source": {
                "user": "james",
                "lastvisited": "2016/01/20 02:03:11",
                "browser": "chrome",
                "offercode": "JB20"
            }
        }, {
            "_index": "db",
            "_type": "users",
            "_id": "AVOiyjIQzUObmc5euUGT",
            "_score": 1.0,
            "_source": {
                "user": "james",
                "lastvisited": "2016/01/20 03:04:15",
                "browser": "firefox",
                "offercode": "JB20,JB50"
            }
        }, {
            "_index": "db",
            "_type": "users",
            "_id": "AVOiyjIlzUObmc5euUGU",
            "_score": 1.0,
            "_source": {
                "user": "james",
                "lastvisited": "2016/01/21 00:15:21",
                "browser": "chrome",
                "offercode": "JB20,JB50,JB100"
            }
        }, {
            "_index": "db",
            "_type": "users",
            "_id": "AVOiyjJKzUObmc5euUGW",
            "_score": 1.0,
            "_source": {
                "user": "peter",
                "lastvisited": "2016/01/20 02:32:22",
                "browser": "chrome",
                "offercode": "JB20,JB50,JB100"
            }
        }, {
            "_index": "db",
            "_type": "users",
            "_id": "AVOiy4jhzUObmc5euUGX",
            "_score": 1.0,
            "_source": {
                "user": "james",
                "lastvisited": "2016/01/19 02:03:11",
                "browser": "chrome",
                "offercode": ""
            }
        }, {
            "_index": "db",
            "_type": "users",
            "_id": "AVOiyjI2zUObmc5euUGV",
            "_score": 1.0,
            "_source": {
                "user": "adams",
                "lastvisited": "2016/01/20 00:12:11",
                "browser": "chrome",
                "offercode": "JB10"
            }
        }]
    }
}

I want to filter out the document based on the user last visited time and get the most recent accessed document of an individual user and then group all the filtered documents based on offer code.

SQL QUERY

select offercode, count(offercode) from users as u1 where u1.ID = (select u2.ID from users as u2 where u2.user = u1.user order by u2.lastvisited desc limit 1) and u1.lastvisited >= "2016/01/20" group by offercode;
Output

{
  "JB20,JB50,JB100":  2,
  "JB10": 1
}

I tried out bool query to filter out the documents based on the time but I don't know to get the most recent visited document and group by offercode in elasticsearch.