How to Replace the WordPress Cron with a Real Cronjob?

I have to update some page everyday with ajax call. Everything is working fine but page content are updated when date changes.
But my problem is I have to make a cron job which is called daily at 12:30 am so I’ve written a cron job:

–> 30 0 * * * /usr/bin/ruby /Users/tamer/scripts/sftp.rb

But when I start my cron job it isn’t working because may be wordpress’s core cron job which is in directory wordpress/wp-cron.php is called before server’s cron is called.

So, I have to stop that wordpress’s cron default job.
Cron job works fine. I tested it with another code. The problem is 110% in wordpress’s default cron job.