How to calculate difference between two datetime in ElasticSearch

Your best best is scripted fields. The above search query should work , provided you have enabled dynamic scripting and these date fields are defined as date in the mapping.

{
  "script_fields": {
    "test1": {
      "script": "doc['acctstoptime'].value - doc['acctstarttime'].value"
    }
  }
}

Note that you would be getting result in epoch , which you need to convert to your denomination.

You can read about scripted field here and some of its examples here.

Here is another example using script fields. It converts dates to milli seconds since epoch, subtracts the two and converts the results into number of days between the two dates.

{
"query": {
    "bool": {
    "must": [
        {
        "exists": {
            "field": "priorTransactionDate"
        }
        },
        {
        "script": {
            "script": "(doc['transactionDate'].date.millis - doc['priorTransactionDate'].date.millis)/1000/86400 < 365"
        }
        }
    ]
    }
}
}