View Current Date, Time and Timezone
RTC = Real-Time Clock, another name is hardware clock, a tiny battery that keeps clocking time even when the system disconnects.
UTC = Coordinated Universal Time, used for synchronizing between timezone across globe.
The warning message means that the Ubuntu reads local system time from RTC (hardware clock) but the disadvantage with RTC is that daylight saving adjustments are not supported. So it is advised to sync RTC with Universal time using the command '- timedatectl set-local-rtc 0'
To change timezone in Ubuntu
# timedatectl list-timezones
# timedatectl set-timezone Region/Location
To change time in Ubuntu
# timedatectl set-time HH:MM:SS
To change date in Ubuntu
# timedatectl set-time YYYY-MM-DD
To synchronize hardware clock (RTC) with Local timezone
# timedatectl set-local-rtc 1
To synchronize hardware clock (RTC) with UTC
# timedatectl set-local-rtc 0