Knowing exact time of power outage looking on system logs files

edit

the last command is what you need.

[michele@benemerita ~]$ last | grep boot | head -1
reboot   system boot  3.19.7-200.fc21. Wed May 27 12:17   still running

and if you put the following command into root's crontab, you know when the system went down

0-59/5 * * * * /bin/touch /root/.system-up

you can use a cron job which runs every minute and then look at the cron log file and look for "holes"

* * * * * nobody /usr/bin/tree

journalctl --unit crond.service

May 17 17:41:33 localhost crond[1383]: (CRON) INFO (RANDOM_DELAY will be scaled with factor 76% if used.)
May 17 17:41:33 localhost crond[1383]: (CRON) INFO (running with inotify support)
-- Reboot --
May 18 21:46:32 localhost crond[1327]: (CRON) INFO (RANDOM_DELAY will be scaled with factor 87% if used.)
May 18 21:46:32 localhost crond[1327]: (CRON) INFO (running with inotify support)
-- Reboot --
May 27 09:23:45 localhost systemd[1]: Started Command Scheduler.
May 27 09:23:45 localhost systemd[1]: Starting Command Scheduler...
May 27 09:23:45 localhost crond[1342]: (CRON) INFO (RANDOM_DELAY will be scaled with factor 23% if used.)
May 27 09:23:45 localhost crond[1342]: (CRON) INFO (running with inotify support)
May 27 10:01:01 localhost CROND[12846]: (root) CMD (run-parts /etc/cron.hourly)
May 27 10:01:01 localhost anacron[12856]: Anacron started on 2015-05-27
May 27 10:01:01 localhost anacron[12856]: Will run job `cron.daily' in 28 min.
May 27 10:01:01 localhost anacron[12856]: Jobs will be executed sequentially
May 27 10:29:01 localhost anacron[12856]: Job `cron.daily' started
May 27 11:01:01 localhost CROND[18943]: (root) CMD (run-parts /etc/cron.hourly)
May 27 12:01:01 localhost CROND[19771]: (root) CMD (run-parts /etc/cron.hourly)
May 27 13:01:01 localhost CROND[21153]: (root) CMD (run-parts /etc/cron.hourly)
May 27 14:01:01 localhost CROND[24560]: (root) CMD (run-parts /etc/cron.hourly)
May 27 15:01:01 localhost CROND[25305]: (root) CMD (run-parts /etc/cron.hourly)

You can write a script to save current time in X seconds interval and run it in background : eg

bash script.sh &

Script :

#!/bin/bash
while (true)
do
date > /var/log/tick
sleep X
done

For a better solution you can create a systemd unit which will run this on system startup, however in that case you have to make sure when the script is run, it archives the old file "/var/log/tick" You can add the following line in script before the loop:

tar cvf /var/log/tick-`date +%d%m%y_%H%M%S`.tar /var/log/tick

This will work in case of power outage, system or application initiated reboot or even a panic.

However if the system is hung this will not be much of help.