How do I find out when I purchased an iPhone?

Purchases from apple.com within the past 18 months can be reviewed by logging in to the site with your Apple ID. Beyond that it’s up to you to keep records of your purchases or your receipts if this is information you need. If products were purchased with a credit card you can review your credit card statements.

For items still in warranty use this link ➡️ Apple - Support - Check Your Service and Support Coverage. The warranty expiration will be 1 year from the date you purchased the item (or the term of your AppleCare contract if you purchased AppleCare).