24 hours notation (Form block 5.7)

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Hi,

I'm trying to build a form with a datetime field.
The time is shown with AM and PM . However I'd like to have a 24 hours notation for the time (European) .

Is this possible in the latest 5.7 version?
In 5.6 we could define a variable, but I've searched for something similar in 5.7 but have not found it.

Jozzeh