How is the value of the "published_at" field determined?

If I don’t add a “published_at” meta tag for the crawler to detect, what value does the crawler set the field do?

In my result details view I see some pages have a published_at value, but I did not define a meta tag on the page to set that value.

Unless the published_at value is defined via meta tag, the field value will default to the last date and time the page was analyzed and updated/indexed by our web crawler.