How to findout the time that the server went down ? or the downtime?

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http://serverfault.com – I have dedicated server running centos 6.0, and apache and the mysql all of a sudden for some reason server has gone down, so i had to reboot the server and it all worked back to normal, However i am wondering how do i get following information, I tried to use "uptime" command this only gives me the time that the server is running what i need to find out is 1. Actual time the server went down ? 2. How long the server has been down ? ...etc any ideas? (HowTos)