Custom Header/Footer cannot be used from GUI
Here on my Debian GNU/Linux system with LXDE/XFCE I encounter different problems with GNOME applications (heavy flickering, overlapping task bar). [EDIT: This was unrelated and wrong, see #457 (comment 901506)]
Gnumeric adds one peculiar problem which seems not to be seen by other users: I cann't choose any custom DATE/TIME format in GUI, I have to edit the xml file manually (see also #456 which has a nice number btw).
I remember to have tried to debug it half a year ago with gdb, but didn't fully understand the gnumeric structure and hadn't the time to report this, although an important error in my point of view. I now just can give the hint that at some point the program opens the custom settings dialog and expects a value to be given to a common place before the dialog is closed (or otherwise). On my computer this doesn't happen, so I don't get a DATE/TIME symbol in the header/footer left/middle/right regions.
If you want to, I can try to analyse it again and make a more detailed description, but probably you much more faster know where the problem is than I can unwind the calling traces.
I unfortunately can't promise to be responsive.
Reproducible in Gnumeric 1.12.44 and 1.12.46