Cronjob to run every 30 minutes

The command in crontab is executed with /bin/sh so you can use arithmetic expansion to calculate whether the current minute modulo 25 equals zero:

*/5 * * * * [ $(( $(date +\%s) / 60 \% 25 )) -eq 0 ] && your_command

cron will run this entire entry every 5 minutes, but only if the current minute (in minutes since the epoch) modulo 25 equals zero will it run your_command.

As others have pointed out, 1 day is not evenly divisible by 25 minutes, so this will not cause your_command to run at the same time every day, but it will run every 25 minutes.

You can do this by defining a job which runs every hour on the hour, and sleeps for a random amount of time before running the command you're actually interested in. In your crontab:

SHELL=/bin/bash

0 * * * * sleep $((RANDOM*3600/32768)) && command

(You need to specify the shell, to ensure that $RANDOM is available. There are other ways of getting a random value for sleep if that's not appropriate.)