Google stores the date of the first time it indexed page on popular sites this date is usually within couple days of the true publish date (on unknown sites it is much less reliable).
To get Google to show the indexed date of page, you do two things:
- Set up search that will only return that particular page by using the site: search term
- Trigger display date by using Search Tools and setting date range that ends with the current day.
- from Web Literacy for Student Fact Checkers by Mike Caulfield, https://mikecaulfield.files.wordpress.com/2017/01/webstrategiesforfact-c...