Elasticsearch monthly index on nested field

Question

How to create a monthly index based on field in Nested document. Example for below document i want to partition based on Joindate. My purging and query search logic is based on that.

{
  "pkClmn": "100",
  "organizationName": "Microsoft",
  "nestedCustomer": [
  {
    "customerName": "John",
    "joinDate": "10-07-2016"
  },
  {
    "customerName": "smith",
    "joinDate": "10-08-2016"
  }
]
}