The purpose of the <abbr>
element is to provide additional information to the readers. In the case of WordPress, if you hover over the date, it shows both the date and the time the post was published. The dotted underline indicates that it is not a hyperlink, but it does provide additional information to the reader.
You can read more about this functionality here.
That said, not everybody who publishes on the web likes this, because most people who read blog posts are completely unaware of what the dotted underline means. You can read more about this here.
If you don’t like it, Kreachure’s code will work beautifully.
Cheers! and remember, GIYF!
Ken