Validity on the ETravelSim Serbia eSIM starts the day you begin using it in Serbia, when your phone first connects to A1's network on arrival. You can purchase weeks in advance, install the QR code at home before flying, and the timer only starts on first connection. Installing at home is the safer option because you'll know before boarding the eSIM is set up correctly. Don't delete the eSIM profile after installation — reinstallation may not be possible.