To add a cron job in Ubuntu.
in console:
crontab -e
Use the line below to perform a task
This will pull a URL every hour on the hour
0 * * * * wget -O - -q -t 1 http://www.example.com/my.php
This will pull a URL every 23 hour and 59 minutes
59 23 * * * wget -O - -q -t 1 http://www.example.com/my.php