When I create a new clock widget
(/usr/share/xfce4/panel/plugins/clock.desktop apparently) it is
initially set to display the time in the format hh:mm. When I open the
settings dialog of the clock, by right clicking on it an selecting
'Properties', I can change the displayed time format and the format of
the mouse-over text. The changes done in the settings dialog are
reflected on the clock widget until I close the settings dialog. Once
I close the dialog, the widget changes to not display anything at
all. When I open the settings dialog again, the format of both the
time and the mouse-over text have been set to a user defined format
without any format string provided. As a consequence, I cannot change
the settings of a clock widget without permanently breaking the clock
or manually fiddling around with xfconf (to avoid using the broken
settings dialog).

This appears to be upstream bug #11527
 (https://bugzilla.xfce.org/show_bug.cgi?id=11527), which was fixed over several
 commits* in the git development repo post 4.13.

I took a stab at backporting the relevant patches, but discovered that they were
written after the GTK3 transition. I don't have the knowledge to easily rewrite
the validater code for GTK2, and merely applying the "clock: Fix invalid
datetime formats appearing in list" patch didn't solve the problem.

* Relevant upstream commits are:

e43248058e67e1cf2725788c894616dc431458cd clock: Fix invalid datetime formats appearing in list
a25624c894e93e1ff354f597975639ef0c2822c2 clock: Add a validator for digital custom formats
989d654b5d6b33e1c4127c740c806420a30b45a4 clock: Fix format validation