Blog of Internet of Things is Bullshit - Hamboigur
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How to use 12-hour clock on Gnome Clocks outside of Gnome

The solution is pretty simple, open dconf-editor and navigate to: /org/gnome/desktop/interface/clock-format and change the value to 12h from 24h.

That's pretty much it. Only posting it here because it's pretty hard to find this information, so maybe a search engine will pick it up =P