Guest set to not sync time, but it still changes (PD9)

Every hour from power-on the guest machine changes time, but I have disabled time sync in VM settings.
It changes time because I have host and guest set to different timezones. I'm aware that this causes problems when using time sync, but I'm trying NOT to use time sync. I have uninstalled Parallels Tools, but still it happens. Why?
I have roughly 30 machines, a mixture of native PC, VMWare VMs and Parallels VMs, and this symptom is only occurring on the 4 Parallels VMs I am running.
I'm aware I'm running an older version of Parallels, PD9. I don't need an "upgrade and see if it goes away" answer. I already know it goes away if I use VMWare. I'd like to make my current installation work correctly.

Can anyone shed any light on how host-guest time synchronisation is achieved? I was confident that it used Parallels Tools, but now I'm not so sure. Is there another service I can disable, a file I can rename etc? Or is it completely managed from outside the guest? Perhaps the Parallels software manipulates the VM from outside.

 

Hi @IanL1 , if you have uninstalled Parallels tools, please install it again as they are necessary drivers which are supposed to be installed inside Windows. Please install Parallels Tools and check if that helps.

 

Not helpful @Arun@Parallels.

This is probably an old problem with PD9 that is fixed now. I would certainly hope so. But it seems to do with the host machine time being in DST. Again, I've posted more details on ServerFault:
https://serverfault.com/questions/908156/find-windows-process-that-is-changing-time-back-1-hour
(It seems this link got edited out of my original post by moderators, also not helpful)

It's a two-part problem. Part of the problem is that it looks like the way PD9 virtualises the Real Time Clock (RTC, the hardware clock) is by using an offset from the host system time. When Windows tries to read the RTC (eg during boot, or for some reason every hour there-after), Parallels gets the host system time and applies the offset. This offset is seen in the config.pvs file stored in <TimeShift>-3600</TimeShift>.

The other part is that for some unknown reason, Windows resynchronises the system time with the RTC on boot, and every hour afterwards.

When I change the guest time and reboot (force Parallels to re-write the config file), the offset is recalculated. But it appears to be calculated using the host system timezone ignoring DST. Eg if host system is Madrid (normally UTC+1, currently with DST it's UTC+2) and guest system is the same, Windows tries to save local time to the RTC. Parallels works out the difference between the time Windows is saving, and the host timezone without DST (UTC+1), and saves -3600 (-1 hr). From then on, every reboot and every hour Windows reads the RTC (host time minus 1hr).

As I said, probably something long since fixed from PD9, but knowing that Parallels virtualises the RTC by saving an offset in the .PVS file would have helped.

 

I have very similar problem MacOS High Sierra+Paralles 13+Windows 10