Include 6 Month record counts in buckets with monthly counts

Hi,

I am using ElasticSearch 6.2 and I have one query to get record counts for Monthly data for particular period. I want to include 6 Month data record count in same bucket whereas 6 Months should not consider current month.

Can you please help me to find a way to get Last 6 Month counts from current month bucket. For example - If monthly data is for Feb-2019, then last 6 Month data count I want is from Aug-2018 to Jan-2019(Means leave Feb-2019) and if monthly data is for April-2019 then 6 month count required will be from Oct-2018 to Mar-2019.

Below is the sample code which currently I am using for getting monthly aggregation portwise -

{ 
  "aggs":  {
  "GroupByPortId": {
		"terms": {
			"field": "portId",
			"size": 10000
		},
		"aggs": {
			"LocalPortBuckets": {
				"date_histogram": {
					"field": "date",
					"interval": "month",
					"min_doc_count": 0,
					"order": {
						"_key": "desc"
					},
					"extended_bounds": {
						"min": null,
						"max": null
					}
				}
			}
		}
	}
  }
}