RTC on board: Mention need for `hwclock` and how to install it

[espinielli]

The documentation in section "RTC on board" mentions hwclock.
In my RPi0W hwclock is not present and needs to be installed with

$> sudo apt install util-linux-extra 

I think it could help PiSugar users to know how to get hwclock
HTH

[JdaieLin]

Thank you for the helpful suggestion. You are right that hwclock is not included in some minimal Raspberry Pi OS installations.

We have updated the PiSugar 3 RTC guide to explain how to install it with sudo apt install util-linux-extra when the command is missing. The guide also includes the related RTC read and write examples.

Thanks again for helping make the setup instructions clearer for other PiSugar users.