Purchase date

Are you asking the date you purchased it from Apple, or the date a previous owner purchased it? Unless you bought it from Apple.com the only record will be your printed or emailed receipt. If you did buy it online log in to Apple.com and you can review 18 months of purchase history.

If you are not the first owner this site can give you some information→Apple - Support - Check Your Service and Support Coverage. If the phone is still in warranty the purchase date will be 1 year before the warranty expiration. If the warranty has expired this link will not provide useful information.

Apple Hardware Purchase History:

1. Click ▶︎ Apple online store.
2. Click on the item from the list that appears.
3. View or print this page (receipt).