Pipeline aggregation with filtering

[AdamBoi (Oleh Adam Dubnytskyy)]

Setup: Elasticsearch v.5.6
Document field called availabilities is the one to perform aggregation on.
The desired aggregation is departure_dates.
Departure date can be calculated as follows: availabilities.arrival_date + availabilities.nights.
I would like to apply mnd field filter to the aggregation also.

Document example:

{
 "availabilities": [
  {
    "nights": 7,
    "arrival_dates": [
    "2019-08-24",
    "2019-08-25"
  ],
  "mnd": [
    201908
  ]
},
{
  "nights": 5,
  "arrival_dates": [
    "2019-08-25"
  ],
  "mnd": [
    201908
  ]
},
{
  "nights": 10,
  "arrival_dates": [
    "2019-09-05"
  ],
  "mnd": [
    201909
  ] 
}
]
}