Uptime question

[yocto]

Hi

Sorry for what seems like a newbie question; I could not find a good 
explanation in the FAQ.
commit d8cfa5db49641f39b63aba2037c6a9e6bd95c0e0
Changed the way uptime works, and in the commit message it is written that:
"Don't read /proc/uptime, because the value it provides does not change when a 
date adjustment occurs."

I actually thought something like the CLOCK_MONOTONIC or CLOCK_BOOTTIME were 
used for providing uptime to the user, but those do not allow date changes.
Hence my question:
I do not understand the relationship between date and boottime. Might make 
sense for VM's, but for non-VM's I do not get it.
Can you help me in understanding ?

/Christian