due to missing dependancy libxfce4util.so.1. I have got /usr/lib/libxfce4util.so.4. Installing without bothering about dependancies (--allow-nodeps) made clear that also libxfcegui.so.3 is missing (.4 exists). 'Creating' these missing libs by soft or hard-links to the existing names does not work, so I am inclined to believe that this is a version mismatch. Could it be that xfce-datetime-plugin-0.3.1 is for an other version of xfce, with thus diff versions of libxfce4util and libxfcegui?
or you might want to try orage-clock plugin, which probably offers the same functionality.