Purchase Date of My iphone 7s

For items from the on-line store go to apple.com, scroll to the bottom, and click on Apple Store Account. --> https://www.apple.com/us/shop/goto/account

For a physical Apple Store purchase you can try contacting the store where you purchased the phone and see if they still have a record of the purchase.

None of the users in here know.

Your personal bank statements are the best way.

or

Contact Apple Support directly. Depending on your region, you may be able to reach them here: Get Support