Then, you want to use a range query: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-range-query.html

You want to build your query string. This is a query string that gives you all the users between the two insertInstants on the user object (inclusive):

 {
 	"range": {
 		"insertInstant": {
 			"gte": 1592231314000,
 			"lte": 1592231315735
 		}
 	}
 }

Omit the gte key if you only want users before a certain time.

Next is to turn it into an API call against the user search API, which means escaping the query's json and turning it into a JSON object suitable for FusionAuth to parse:

curl -XPOST -H 'Content-type: application/json' -H "Authorization: $API_KEY" 'https://HOSTNAME/api/user/search?' -d ' { "search" : { "query" :  "{\"range\" : { \"insertInstant\" : { \"gte\" : 1592231314000, \"lte\" : 1592231315735}} }"}}'

Finally, after testing to make sure you're getting the users back you want, you can run the bulk delete API, as documented here: https://fusionauth.io/docs/v1/tech/apis/users#bulk-delete-users

Tested with Elasticsearch 7.