This is a very basic operation on a Linux system, but I have to interact with it so rarely since I run either chronyd or ntpd on all of machines. Occasionally, I find a device that needs a helping hand.
On a device with a known good time (or approximate enough):
This will get you the current unix timestamp. On the target system needing
updating as root (replacing
<unix timestamp> with the results from the last
date --set="@<unix timestamp>" hwclock --systohc
Ensure both clocks are updated:
date hwclock --show