If by timestamped you mean you'd like the time to be written in the file, I would recommend using the snipmate plugin. For example, in my ~/.vim/snippets/vimwiki.snippets file, I have a date snippet that expands the full date & time using: `strftime("%F")`
On the other hand, if you want the filename itself to be timestamped, that would take some more work. I would recommend submitting an issue on the vimwiki site.
- S.