When does the validity period start — at purchase or when I land?

Validity on the ETravelSim Taiwan eSIM starts the day you begin using it in Taiwan, not the day you buy or install it. You can purchase weeks in advance, install the QR code at home before flying, and the timer only starts when your phone connects to Taiwan Mobile on arrival. 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.