Did you know that every year you purchase an iPhone comes with a one-year limited warranty? This applies only to Apple products and is officially called the "Apple One-Year Limited Warranty". One year from the date of purchase at the first store, guarantees against material and manufacturing defects (so-called initial defects) during normal use according to Apple issued guidelines, including accidents and improper handling not.
This one-year warranty starts when you first activate your iPhone. If you connect to the Internet without signing in with your Apple ID, you can tell Apple's server that your iPhone has started working, and you can start a one-year limited warranty.
When I tried to check the date, I had to guess it based on the documents that were used to purchase the iPhone, but since iOS 12.2, it can be checked on the iPhone. There is no loss in knowing when the warranty expires because it is valuable information for determining whether free repair is possible and when to sell.
The procedure is as simple as opening the screen in the order of “Settings” → “General” → “Information”, and check the “Limited Warranty” item. The expiration date should be displayed as "Expires: 2020/09/19". If you tap this item,
Since the warranty period is one year from the start of use, in this case you can see that it started using from September 20, 2019. Estimated as a predecessor, but because it is displayed based on information received by Apple's server, it is good information to trust.