Kibana: timestamp mappings and no results


I'm using Java API [1.5] to index high amount of data in Elasticsearch, and I'm defining mappings for several fields (some needs to be "not_analyzed" and one is a timestamp).

My problem is that all is correctly indexed in Elasticsearch and available using curl and Java API, but Kibana can't find results in the "discover" and "visualize" views, even if it correctly find my index and every fields in the "settings" view.

I'm converting my date in a formatted timestamp String in my Java code :

Calendar calendar  = Calendar.getInstance();
SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMddHHmmss");
String timeToString = sdf.format(calendar.getTime());
hospitalMessage.getActivity().put(FIELD_TIMESTAMP, timeToString);

In debug mode, a value of my string variable is "20150413155815", which correspond to my format.

Then I index my Java Object using Jackson to send it in a JSON file to Elasticsearch.

I've also declared a mapping for this timestamp field as following :

"timestamp" : {"type" : "date", "format" : "yyyyMMddHHmmss"}

Doing this, no results are found in Kibana and both Elasticsearch and Kibana consoles aren't showing any error or exception.

When I change the mapping to a simple string value (after cleaning all indexes and mappings in elasticsearch) everything works fine :

 "timestamp" : {"type" : "string"}

I'd really like to use my index with time-based events in order to provide some visualizations showing events per day (i.e count per day in a bar graph) and I don't understand what is the problem with my mapping.

Intallation :

Elasticsearch 1.4.4
Kibana 4.0
Java API 1.5

Thanks for answers,