There is no tag for datetime 0.5.0 in the repo.
http://svn.xfce.org/index.cgi/xfce-goodies/browse/xfce4-datetime-plugin/tags

Fedora 9 includes version 0.5.0:
xfce4-datetime-plugin-0.5.0-4.fc9.i386

$ rpm -q --changelog xfce4-datetime-plugin | head -15
* Mon Feb 18 2008 Fedora Release Engineering <rel-eng@fedoraproject.org> - 0.5.0-4
- Autorebuild for GCC 4.3

* Sat Aug 25 2007 Christoph Wickert <fedora christoph-wickert de> - 0.5.0-3
- Rebuild for BuildID feature
- Update license tag

* Sat Jun 09 2007 Christoph Wickert <fedora christoph-wickert de> - 0.5.0-2
- Multilib fix for desktop file (#228168).

* Sat Feb 24 2007 Christoph Wickert <fedora christoph-wickert de> - 0.5.0-1
- Update to 0.5.0.

* Mon Jan 22 2007 Christoph Wickert <fedora christoph-wickert de> - 0.4.1-2
- Rebuild for Xfce 4.4.