Classified Ads exposes the ad expiration "extra-field" as a CTools content type.
In practice, this means you can add it to a custom-built page in Panels just like core Fields.
The field has two settings:
link
defines whether or not the expiration "field" should
link back to the node URL.format
defines the format under which the expiration
"field" should appear: either as a remaining lifetime, a remaining
percentage, an actual expiration date, or using the default formatter
used for the normal node view.