Below the main editing text area there is Advanced Properties
The date box is the first and is named "issue date"

Thanks for the reply. That is indeed where it used to be but not anymore.

Under Advanced Properties I have:
URL "filename"
Tags
Excerpt

and nothing else...

What happens when you select "Simple" post instead of "Expert" post?
In Simple mode the same date edit function is also available

Ok, I found the problem.

http://manual.b2evolution.net/User_levels

I read the 'deprecated' part so I didn't pay attention to the rest.

Setting the level to 10 fixed the problem.

Thanks for your help.

Good news :)