I'm facing a situation now: There are two batches of nodes in an ES cluster. One batch loads 'user.timezone=Asia/Bangkok' from etc/timezone at startup, while the other batch loads 'user.timezone=America/Bogota'. Even though the node physical times are kept consistent using the same NTP service, I'm not sure what actual impact the nodes with different time zones will have on the cluster. Version: 6.8.1
Data indexed in Elasticsearch will always be in UTC, it doesn't matter the timezone of the server, this will probably only impact the time in the logs generated by the server itself.
Also, you have a bigger problem, version 6.8.1 is ancient and should not be used anymore, you need to look in upgrading it.
Hmm I think the node's local timezone will affect how timezone-free timestamps are parsed and converted into the internal representation which uses UTC. So that would be confusing if you rely on that feature anywhere.
I'm not aware of anything that'll actually break in a cluster with nodes with a mix of timezone values, but this is definitely something we do not test so I can't recommend it. Experience has repeatedly shown me that running servers in local timezones (i.e. not in UTC) just causes pain in the long run. My best advice would be to switch your whole cluster to use UTC.
And yes the 6.x series is irresponsibly out of date these days, you need to upgrade as a matter of urgency.