Actually i want all unpublish nodes to be delete automatically whose created date is 2 days ago. I can write php script for that and also include that code in hook_cron api.
but i am confused that my site is set to be run cron after every 3 hours and i want that php script to be run after 24 hours. can i set specific cron time for specific job. i dont want change the site cron job time.
Set system cron to every 5 minutes, and use Elysia Cron to manage particular tasks:
Elysia Cron extends Drupal standard cron, allowing a fine grain control over each task and several ways to add custom cron jobs to your site.
[NEW IN 2.0] Set the timings and frequencies of each cron task (you can run some jobs every day at a specified hour, other only monthly and so on...). For each task you can simply choose between some frequently used options ("once a day", "once a month" ...), or use a powerful "linux crontab"-like syntax to set the accurate timings. You can even define your frequently used options to speed up site configuration.