You'd need to integrate the code example from the link above, specifically the div structure with php date code, into a filter for use on the postmeta. If you dig into the forum search looking for code examples of doing postheader filter or postmeta filter, you should come across a couple workable examples to help you wrap your head around what you need to do.
If you look in /thematic/library/extensions/content_extensions.php, and find the code block dealing with the postheader postmeta, You'll see how the time/date information is added to a default thematic post, and from there give you an idea of how to structure your filter to replace the default date style with your custom stuff.