My English version of Windows 10 and Ubuntu 16.04 could not agree whether my hwclock is UTC or not. So booting either Window or Linux changed my hwclock and then the time in the other OS was wrong. So I changed my hwclock to UTC and told Windows and Linux that my clock is set to UTC:
Windows:
Windows Registry Editor Version 5.00 [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\TimeZoneInformation] "RealTimeIsUniversal"=dword:00000001
Linux:
Open the file (as superuser)
/etc/default/rcS
and add or change UTC=yes