Side question: does auto clean process is still using mail receiving date instead of time when message was placed in trash for after X day removal (this is critical in situation where user is moving some mails to trash and this messages are being almost right away deleted from it since they where delivered more than X days to users mailbox)? New file format in v17 (or v16?) was a suggestion where this can be made and I'm wonderin if this has been implemented.

Hate the Windows time service, always full of issues. We use a local time server (SyncServer S80) with https://www.meinbergglobal.com/english/sw/ntp.htm as the NTP client. The NTP client has worked for us for ~10 years, never had an issue. Much more reliable than the Windows Time service.

Yes, we disabled windows time service and replaced it with http://www.timesynctool.com/

No more time jumping issues since.

Edit: SyncServer S80 seems great. Maybe we should buy one to use instead of public ntp servers.

Yep, quite a while back we had a big issue with one of the public NTPs we used going nuts. One of our clients uses software that relies on time being damn near perfect and it messed their system up badly when this happened. So we invested in one of these, it is the primary time for our network, but we also use NIST's time servers as a backup (they are very close to our DC, so we get a ~5ms response time).