Solution 1:

I had the same problem and found a workaround.

Right click the clock panel (or access from panel preferences) and set the configuration you want but do NOT close the properties window. Instead, while leaving it open, logout and log back in. The clock should reappear with the preferences you set. For some reason closing the properties window seems to reset the settings to blank custom fields, which makes the clock disappear.

Solution 2:

Best workaround as commented by https://askubuntu.com/users/513351/ali-caglayan but as an answer:

Change the properties but don't close the window.

then restart the panel,

xfce4-panel --restart

Solution 3:

My way around this was to set it to what I want. and without closing the clock properties.

I open terminal and run this command

killall xfce4-panel

followed by this command

xfce4-panel & 

Then you can close terminal (which may make the panel refresh again). But my settings I picked for my clock remained. The properties thing is just all kinds of borked up. It was broken like this while it was still a beta. Figured it be fixed by the time it it was released. But nope, clearly no.