I've checked and my BIOS clock stays correct so it can't be the CMOS battery.
Also when I boot Windows 10 and the clock is an hour behind if I got and sync the clock with the time server it shows correctly.
So I'm not sure what could be causing the issue.
Best Answer
Had this issue on my laptop and regular PC (both dual boot Windows 10 and Xubuntu) - the time was always 1 hour behind.
Before I give my solution, just to be clear;
What worked for me, was the following; change the "Internet time" settings to time.nist.gov.
Time is now shown correctly again, even after restart.
If you are still having issues with time after these steps verify the Windows Time Service is running (w32Time)
This should ensure that if you are using NTP (Internet Time) then you should be able to sync your clock on startup.