[CentOS] Problem with cron

I have a root cron job that powers down my server every day at 1am and 
6pm.  The output of '# crontab -l' is shown below.

* 1,18 * * * poweroff

Last night, after the server powered down at 6pm, I decided I wanted to 
use the server so I started it with the power button.  The server, after 
a minute or so, powered itself down.  This behaviour happened repeatedly 
until I waited past 7pm. Then the server did not poweroff.

Apparently a cron job that executed correctly at 6pm was executing 
minutes past 6pm when the server was restarted. This is totally 
unexpected behavior.

Here is the output of the cron log file.  The name of the server has 
been redacted.

# grep -i 'Feb 22' cron-20140223 | grep -i poweroff
Feb 22 18:00:01 xxxxxxxxxx CROND[2875]: (root) CMD (poweroff)
Feb 22 18:12:01 xxxxxxxxxx CROND[1894]: (root) CMD (poweroff)
Feb 22 18:16:01 xxxxxxxxxx CROND[1893]: (root) CMD (poweroff)
Feb 22 18:18:01 xxxxxxxxxx CROND[1896]: (root) CMD (poweroff)
Feb 22 18:22:01 xxxxxxxxxx CROND[1915]: (root) CMD (poweroff)
Feb 22 18:25:01 xxxxxxxxxx CROND[1919]: (root) CMD (poweroff)
Feb 22 18:34:01 xxxxxxxxxx CROND[1890]: (root) CMD (poweroff)

Is there a fix for this behavior?

Thank you,
Joe