GSM SIM808 power supply and grounds + power sleep mode

Hello again,
I would like to ask something very simple that i cant find answers for it anywhere. I am using a gsm sim808 hw-589 connected to an atmega328P standalone. both are powered with 4 li-on 18650 3,7V protected batteries coneccted in parallel. Basically there is 1 pair of wires (+,-) from the batteries that goes to the li-on battery pins on the gsm and another pair of wires that goes through a step up module 5V and then to the atmega. My question is this i know that the gsm and the atmega have to be connected to RX and TX and ground but is that ground really necessary or not? I am asking cause i accidently removed the ground wire from the battery that connects to the gsm but the gsm was still working so i guess that it was cause of the ground connected to the atmega?
My second question dunno if it can be anwered here but i will try. I want to send data from the gsm every 12 hours so to save battery the atmega goes tyo sleep mode but for some reason it doesnt open every 12 hours but in every circle it adds 1 more hour, can this be cause of a bad crystal or what else can cause this?
Thank you in advance.