Kernel version (static compile time info due to reproducible builds)

AFAIK, uname -v and /proc/version shall give the compile time of the kernel. Am I right? It now gives times close to last git check-in time.

Likely so.
The goal is to be able to compile identical "reproducible" binaries later, so the file times are adjusted in a definite way (to the time of the last git commit). Likely also the kernel version time is set the same way.

Most likely the behaviour is due to;a=commitdiff;h=be6acba4dc5c9013a8f61fb2e9d2dbb976126aa8;hp=ad8169546cbcb5b2ed59cef332bb6b1dc5e389ef

and uses the time set by;a=commit;h=82522dbaee8cd5f94267ebf465e36d594e4d2d7a

In my own personal build I want the date/time to be the actual compile time, so I override the default SOURCE_DATE_EPOCH logic by setting the current date/time to a file "" in the buildroot root dir before running "make".

# Override git/svn timestamp after r48583-48594, set initial clock to now
date +%s >

Now that you mention this, I recall a presentation in FOSDEM 2016 about
that topic. I just didn't realized that this was already happened.