Hello fatinaz,
Thank you for the post. I understand you're seeing "purchase date not validated" when you enter your iPhone serial number. I'm glad to see if I can provide some helpful information for you.
For clarification, are you referring to entering your serial number on this site? Check Your Service and Support Coverage
The following resource provides some information about this: Correct the purchase date or expiration date for your AppleCare agreement
"If your coverage expiration date is incorrect
If the estimated expiration date of your Telephone Technical Support, Limited Warranty, or AppleCare agreement for your serial number is incorrect, please contact us. You will need to send the original sales receipt of your product to Apple so that we can update your purchase date. A sales receipt with the receipt number, product description, original date of purchase, price, and reseller details constitutes a valid proof of purchase.
To update your purchase date, send Apple your proof of purchase"
I hope that helps! Enjoy your day.