Incorrect month in Elasticsearch date_histogram - elasticsearch

Question

My Document looks like below:
{
"_index": "rep_cdr",
"_type": "doc",
"_id": "TaArd2YBDRXNehCp7GmW",
"_score": 1,
"_source": {
"level": "info",
"#version": "1",
"thirdPartyTime": 139,
"date": "15-10-2018",
"time": "15:00:59",
"reqId": "25718d6e-b8ef-438d-8218-1a8726c6c816",
"TAT": 1574,
"message": "",
"thirdPartyErrorDescription": "",
"#timestamp": "2018-10-15T10:00:59.146Z",
}
}
And I am running following query:
GET rep_cdr/doc/_search
{
"size": 0,
"aggs": {
"datewise": {
"date_histogram": {
"field": "date",
"interval": "day"
}
}
}
}
I am getting below result:
{
"aggregations": {
"datewise": {
"buckets": [
{
"key_as_string": "15-01-2018",
"key": 1515974400000,
"doc_count": 8
}
]
}
}
}
Index mapping is as below:
{
"rep_cdr": {
"aliases": {},
"mappings": {
"doc": {
"dynamic_date_formats": [
"DD-MM-YYYY",
"HH:mm:ss",
"yyyy-MM-dd'T'HH:mm:ss.SSS'Z'"
],
"properties": {
"#timestamp": {
"type": "date",
"format": "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'"
},
"#version": {
"type": "text",
"fields": {
"keyword": {
"type": "keyword",
"ignore_above": 256
}
}
},
"TAT": {
"type": "integer"
},
"date": {
"type": "date",
"format": "DD-MM-YYYY"
},
"level": {
"type": "text",
"fields": {
"keyword": {
"type": "keyword",
"ignore_above": 256
}
}
},
"message": {
"type": "text",
"fields": {
"keyword": {
"type": "keyword",
"ignore_above": 400
}
}
}
"reqId": {
"type": "text",
"fields": {
"keyword": {
"type": "keyword",
"ignore_above": 256
}
}
},
"response": {
"type": "keyword"
},
"thirdPartyErrorDescription": {
"type": "text"
},
"thirdPartyTime": {
"type": "integer"
},
"time": {
"type": "date",
"format": "HH:mm:ss"
}
}
}
},
"settings": {
"index": {
"creation_date": "1539236694553",
"number_of_shards": "3",
"number_of_replicas": "1",
"uuid": "BYDQOhY_TbWhuqMAOA3iNw",
"version": {
"created": "6040099"
},
"provided_name": "rep_cdr"
}
}
}
}
The "key_as_string" gives me wrong month. In document the date field has value "15-10-2018" but "key_as_string" gives me "15-01-2018". I am using elasticsearch version 6.4. What could be wrong?

Answer 1

Your date field format is set to DD-MM-YYYY where D is day of year as mentioned on https://www.joda.org/joda-time/apidocs/org/joda/time/format/DateTimeFormat.html. Change your date format to dd-MM-yyyy instead and it should work as expected.
What you are seeing in response is 15th day of the year i.e. 15-01-2018