Dear @Dzmitry.
Im running version 7.9.0.
Server time seems to be ok.
tecnetadmin@ubuntu-elk:~$ sudo dpkg-reconfigure tzdata
Current default time zone: 'America/Argentina/Mendoza'
Local time is now:      Sat Sep 26 11:03:37 -03 2020.
Universal Time is now:  Sat Sep 26 14:03:37 UTC 2020.
Here is how I build the time string to insert on elastic.
 [leo@arch ~]$ cat test.sh 
    a=$(date "+%F %T")
    b="${a/ /T}.000Z"
    echo $b
    [leo@arch ~]$ bash test.sh 
    2020-09-26T11:11:08.000Z
Data is geeting stored ok , for example using this littel script:
root@UBUNTU-SMOKE:/opt/lease_parse# cat date.sh 
#!/bin/bash
a=$(date "+%F %T")
b="${a/ /T}Z"
echo "$b"
A=1111
C=3333
curl -XPOST "http://172.30.6.113:9200/dhcp_leases/_doc/" -H 'Content-Type: application/json' -d'{"router":"lujan","current":'$A',"previous":'$C',"timestamp":"'$b'"}'
inserting a test document:
root@UBUNTU-SMOKE:/opt/lease_parse# bash date.sh 
2020-09-26T11:42:44Z
{"_index":"dhcp_leases","_type":"_doc","_id":"n8rdynQBT4bdce0nv8nQ","_version":1,"result":"created","_shards":{"total":2,"successful":1,"failed":0},"_seq_no":20,"_primary_term":2}root@UBUNTU-SMOKE:/opt/lease_parse# 
Then I can see on kibana (discover) , last inserted register:
Sep 26, 2020 @ 08:42:44.000                                  (wrong time here)
router:
    lujan
current:
    1,111
previous:
    3,333
timestamp:
    Sep 26, 2020 @ 08:42:44.000
_id:
    n8rdynQBT4bdce0nv8nQ
_type:
    _doc
_index:
    dhcp_leases
_score:
    - 
But if I look on elastic , i have:
GET dhcp_leases/_doc/n8rdynQBT4bdce0nv8nQ
wich returns:
{
  "_index" : "dhcp_leases",
  "_type" : "_doc",
  "_id" : "n8rdynQBT4bdce0nv8nQ",
  "_version" : 1,
  "_seq_no" : 20,
  "_primary_term" : 2,
  "found" : true,
  "_source" : {
    "router" : "lujan",
    "current" : 1111,
    "previous" : 3333,
    "timestamp" : "2020-09-26T11:42:44Z"                        (time is ok)
  }
}
ok thats the complete process.
Please let me know if it is clear or something else to check.
Perhaps need to change the timestamp string format ?
Thanks for your help!!