I want to thank you for the answer, and for having clarified an important aspect for which initially, even if I set the type to datetime, I could not get what I wanted: the formatting has effect in the browser.
I managed to format the date even better than I originally wanted.
Very briefly, below is my solution, if in the future it can be useful to some beginner like me.
I set the format on the item label, in Paper UI.
Below is the thing channel associated with the Item. I named it “LastUpdate” (it contains the time string provided from the sonoff)
This is what I see in the brower
I’ve found more informations about Datetime formatting here:
Time and Date Formatting in openHAB
Java Class Formatter