As i used it after alarm set i power down it automatically on.. datasheet says use a capacitor or give a 3 volt ..but i did not use capacitor or 3 volt but still RTC running after power down .
If you don´t do like in the datasheet... it may work, but not reliable.
* So maybe tomorrow it doesn´t work anymore.
* Or maybe with another SIM900 it doesn´t work.
* or maybe with higher or lower temperature it doesn´t work anymore.
It´s on you how you design it. Whether you want it to work "by accident" or you want it to work reliably.