In 4.9+ xfce4-panel there are 2 vertical modes - "deskbar" mode, in which plugins are laid out horizontally, and vertical mode, where plugins are vertical.
Datetime plugin should follow this convention.
Created attachment 4108
A patch implementing this feature.
The patch should be compatible with 4.8 libs.
In addition to the orientation change, the patch also reduces "spread" of labels in wide panels. This is somewhat unrelated (sorry) but it was convenient to implement it at the same time as the orientation fix.
Your patch was very similar to the one in 8926, but that one used an xfce_hvbox so i took the latter. other than that, should be feature-equivalent..
*** This bug has been marked as a duplicate of bug 8926 ***