How to tell Ubuntu that hardware clock is local time?

In Ubuntu releases that use systemd the command to change time to local and update the clock right away is

timedatectl set-local-rtc 1 --adjust-system-clock

If you run timedatectl, it will show with a warning

               Local time: ...
           Universal time: ... 
                 RTC time: ...     
                Time zone: ...
System clock synchronized: yes                         
              NTP service: active                      
          RTC in local TZ: yes  

Warning: The system is configured to read the RTC time in the local time zone.
         This mode can not be fully supported. It will create various problems
         with time zone changes and daylight saving time adjustments. The RTC
         time is never updated, it relies on external facilities to maintain it.
         If at all possible, use RTC in UTC by calling
         'timedatectl set-local-rtc 0'.

This warning doesn't mean that it is set to 0. If it was set to 0, timedatectl status says:

RTC in local TZ: no

and there is no Warning.

RTC in local TZ: yes

is preferred, if you're dual booting. The suggested command is for switching RTC back to "no".