What it shows: date the device was linked to that Mi Account — often close to activation date. Limitations: If the seller/previous owner registered the phone, the date reflects that prior registration.
This is completely normal. If you bought the phone, let it sit in its box un-opened, or didn't immediately connect it to Wi-Fi or cellular data upon unboxing, the system won't record the activation until it communicates with Xiaomi servers. how to check activation date of xiaomi phone
Xiaomi typically offers a 24-month (2 year) warranty from the activation date. What it shows: date the device was linked