Command at /etc/crontab not run at reboot?

I have used the crontab to run a command that repeats every 30 minutes. From the tutorials, I written the following command to /etc/crontab

*/30 * * * * root /usr/bin/python /home/pi/do.py>>/home/pi/output

As per the tutorials found on google, it should run for 0,30,60…minutes. But it won’t run at reboot. However, it runs for every 30 minutes after reboot.

How can I configure the same for a run at reboot also?