Check the original purchase receipt or invoice
Look in your email for the order confirmation
Check your online account history with the retailer or carrier
Review your bank or credit card statement for the transaction date
Check the phone’s warranty or activation date
Look at the device’s purchase history in your Apple ID or Google account
Contact the store, carrier, or manufacturer with the serial number or IMEI
Check any trade-in, financing, or installment records
Look for the date you first activated the phone on your account
Review shipping confirmation emails if you bought it online