This Add-on tries to find out when exactly a website or an article on a website was published and last edited.
There is no guarantee, that a web page provides any visible or hidden information about when it was published / last updated!
"Published Date" uses around 40 different ways to try to find out when the web page you are visiting was published and when it was last edited / updated. Therefore, there can be multiple possible (and even different) results.
Just visit a website and click on the Add-on icon!
The Firefox extension can be installed directly via the Firefox extensions site.