This guide outlines the most reliable methods to determine the exact activation date of a Xiaomi device. Method 1: Using the Xiaomi Product Authentication Site
If you are meeting a seller to buy a used Xiaomi, Redmi, or POCO: how to check activation date of xiaomi phone
If you cannot turn on the phone (e.g., you are buying it used and want to check before powering it up), or if the internal tools are buggy, you can use the IMEI number. This guide outlines the most reliable methods to