SUMMARY
POTD technically does change every day, but not in the way that I (and maybe others) would expect. 

STEPS TO REPRODUCE
1. Set one of the monitors' background to Bing POTD
2. Wait until the next day
3. Sadly look at the same picture as yesterday, while your custom text display plasmoid shows you the description of what you should be looking at :(

OBSERVED RESULT
POTD may be stale (noticed particularly with the Bing POTD), requiring a `killall plasmashell && plasmashell` to get it to update

EXPECTED RESULT
POTD is new every morning when I come into the office and logon to my computer (I would expect that it changes at 12AM local time, or whenever the POTD provider has updated their picture)

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 5.14.5
Qt Version: 5.12.1
KDE Frameworks Version: 5.54.0
Kernel Version: 4.20.7-arch1-1-ARCH
OS Type: 64-bit
Processors: 8 × Intel® Core™ i7-8650U CPU @ 1.90GHz
Memory: 15.4 GiB of RAM


ADDITIONAL INFORMATION
If there were a way to force an update via dbus command or something, that would be an acceptable workaround, too

Note, I don't know very much about KDE programming, but I'd be happy to help where possible. In my estimation, the checkDayChanged() method (https://cgit.kde.org/kdeplasma-addons.git/tree/dataengines/potd/potd.cpp#n178) will have to be modified to get this working as expected.