Perform time arithmetic operation on two time fields in Grafana+ Elastic Search

Grafana
1

Elastic Search document:
_index:a
_type:b
_id:c
{
…
“timestamp_1”: “2019-05-03T11:27:22Z”,
“timestamp_2”: “2019-05-03T13:31:12Z”,
…
}

How can I write query in Grafana to subtract these 2 timestamp fields to become another field?