But there's still the CSS issue with the DEV edition :-)

Can you post a screenshot, so that we are on the same page?

Sure!
From the 2.8.0 DEV:

imgur.com/a/MSVhO

This might help :)

github.com/getk2/k2/commit/0166ec1c9f8eed6188f84f48032a6e82d3a006fc

Looks a lot better, thanks!

How can I change the time picker to 24 hour clock?

You would have to edit some core files in order to change the initialisation script ( chmln.github.io/flatpickr/examples/#time-picker )

I would suggest the following, posting this as a feature request on github so we implement it on the core.

Hi and sorry if I go back to this topic
Since yesterday, I'm using Joomla! 3.7.5 Stable (ita) and k2 2.8.0.
After updating, I can not change the date of the articles: they can not be edited.
I checked, with some difficulty because I'm not really expert, the various suggested changes and I think they're all already active.
Can you help me solve this bug?
What useful information can I provide?
Thanks in advance

Can you send me a screenshot of the issue?

Thanks for replying.
As you can see from the screenshot, the date is not editable

Have you cleared all caches after the upgrade? I mean Joomla cache, your browser's cache, server cache if there is any and Cloudflare cache if you use Cloudflare. I had similar issues, where the calendar looked funky and it didn't work well. Cache was the issue.