How to check the date of purchase of my iPhone?

The serial number you posted in the first post was edited out as will the second post. You are posting personal information on a user to user only forum. No one here should ever know your serial number, nor would knowing it help in any way. We don't have access to anything dealing with the purchase of your phone. You should have kept a copy of the purchase receipt when you bought the phone. That is how you can know the date of purchase.

You can check your warranty coverage here, which typically coincides with the date the phone was purchased --> Check Your Service and Support Coverage - Apple Support

If the coverage has lapsed, however, it won't help you much in this regard.