I can honestly tell you that NO ONE, except “possibly” google (and Gary Illyes in particular) has the actual answer to this question.
You see, for the past several years there has been a really big problem with google and dates. I particular, when something was actually posted.
I’ve spoken with Gary about this issue, and have shown them examples. An example would be me making a new post on my blog, it gets crawled and indexed within minutes. Then google shows, in the serps, that it was posted 8 hours ago. It was not posted 8 hours ago, the breaking news I posted about didn’t even happen 8 hours ago.
Google still, several years later, tends to have a problem with dates. They may have fixed it, but that would be recently. I have not looked if it’s still an issue.
No one knows for sure how google determines the dates on posts or pages. If they told us then we would just change the server date and time, or add the Daye and time to the post to manipulate it. You see, if we knew how to manipulate it, then there could be serious legal consequences for google and site owners. Such as who posted content first, one site owner could sue another if the other site owner was scraping content and someone could prove the time it was posted.
I’ve actually even see google use a date from an embedded YouTube video on the page, and the video wasn’t even theirs.
I could go on and on about dates on posts, it’s something I’ve watched and studied, and even talked to Gary about.