Standardiser les heures avec l'attribut datetime HTML5: What's change

Standardiser les heures avec l’attribut datetime HTML5
I don’t understand the aim of this exercice because after I put the solution nothing change in the view windows.

The key to understanding the exercise is in the text of the lesson.

“A datetime attribute holds a valid format of that date. This is the value accessed by assistive devices. It helps avoid confusion by stating a standardized version of a time, even if it’s informally or colloquially written in the text.”

In the first two sentences it’s telling you that by using the datetime attribute you’re helping someone using something like a screenreader.
It then goes on to say that even if you have it written in the text this attribute will help avoid confusion.
So while you will not see any difference in the preview window, someone using assistive technology will better understand.

Hope that helps.