VM: Set guest hardware time for qemu from libvirt

Dear community,

I have a virtual machine with windows 10 installed. I tried to set the guest hardware time for qemu from libvirt to a specific value:

<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
...
<qemu:commandline>
  <qemu:arg value='-rtc'/>
  <qemu:arg value='base=1970-01-01T00:00:00'/>
</qemu:commandline>

But that doesn't work. I also tried to change the clock tag to something like:

<clock offset='variable' adjustment='-500000' basis='utc'>
  <timer name='hypervclock' present='yes'/>
  <timer name='hpet' present='no'/>
</clock>

Here is the problem that the date / time is printed correctly in the Log "-rtc base=2019-09-23T14:25:47 \" but not set up corrently in operating system and I have to change these value in the *.xml all the time. I figured out, if I start the virtual machine and click on "Edit" the value changed to another value. In my case to -500001 or different.

My plan is, an external hardware (arduino + dcf77 modul) and the corresponding software receives the current time via dcf77 longwave time signal and updates the time of the operating system over the air.

Does someone of you tried this? Can someone help me out or can give me some useful tips or links?

That would be great!

Thanks a lot,

T65