Hi Capo.
Alas I think that won't work the way you want.
Alas the RTC is only that. It keeps time.
What you may need to do is - and I am REALLY GUESSING here - is see if you can put the arduino into a sleep mode and set one of the pins as an interupt input.
When this pin is stimulated, it wakes up, does what you want and goes to sleep again.
That would require an external circuit timing and sending a pulse every 30 minutes.
Alas: I fear that the external circuit doing this may take more power than is saved with the arduino in sleep mode.
As you may see, I too am new so may be way off, but using the RTC won't work to do what you want.
Good luck though!