How to get (semi)exact uptime values for processes?

If you look at the ps faux listing, you'll see a bunch of values:

walter   27311  0.8  1.8 5904852 621728 ?      SLl  sep06  61:05  \_ /usr/lib/chromium-browser/...
walter   27314  0.0  0.2 815508 80852 ?        S    sep06   0:00  |   \_ /usr/lib/chromium-brow...
walter   27316  0.0  0.0 815508 14132 ?        S    sep06   0:01  |   |   \_ /usr/lib/chromium-...

That second value (27311) is the PID, the tenth (61:05) how much CPU time has been spent. And the ninth (sep06) is when the process started.

You can shorten the listing:

$ ps -p 27311 -o pid,start_time,bsdtime
  PID START   TIME
27311 sep06  61:05

And you can also get more granularity (and readability) out of the times:

$ ps -p 27311 -o pid,lstart,cputime
  PID                  STARTED     TIME
27311 Wed Sep  6 14:48:35 2017 01:01:05

Internally, ps(1) fetches the data from /proc/[pid]/stat. Read about its format in man 5 proc.

For instance, to fetch the start time, the following shell script would do the trick, by fetching the 22nd value from /proc/[pid]/stat. The process start time is stored as seconds-since-boot value:

$ get_proc_starttime() {
  PID=$1
  SYSUP=$(cut -d. -f1 /proc/uptime)
  PIDUP=$((SYSUP-$(awk 'BEGIN{CLK_TCK=100}{printf "%lu",$22/CLK_TCK}' /proc/$PID/stat)))
  date -R -d "-$PIDUP seconds"
}
$ get_proc_starttime 27311
Wed, 06 Sep 2017 14:48:36 +0200

Apropos the time in seconds-since-boot format. That's also the times you see in the default dmesg(1) output:

$ dmesg  | head -n1
[    0.000000] Initializing cgroup subsys cpuset

If you want to make sense of the times (and don't have a nice /var/log/kern.log to look at), you could translate the time-value like this:

$ SYSUP=$(cut -d. -f1 /proc/uptime); dmesg |
  sed -e 's/^\[\([^.]*\)\.[^]]*\]/\1/' |
  while read x; do echo "[$(date -R -d "-$((SYSUP-${x%% *})) seconds")] ${x#* }"
  done | head -n1
[Tue, 25 Jul 2017 16:04:59 +0200] Initializing cgroup subsys cpuset

But, you won't need that; dmesg itself also has an appropriate flag as well: dmesg -T

$ dmesg -T | head -n1
[Tue Jul 25 16:04:58 2017] Initializing cgroup subsys cpuset

process uptime linux