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
