Two way to same the same thing

I am currently studying survival phrases in section 2 and found there 2 ways to say the same thing (see below).  I get confused when there are 2 ways to say the same thing. Which of the two expressions below is most commonly used?
As you know, there are many more like it in this survival lesson.

le otto di sera / le venti

Hi William,

Both expressions are extremely common and they are totally interchangeable, both in formal and informal contexts. Literally they mean "the eight in the evening / the twenty". This is because on a 24-hour basis 8 pm is the 20th hour.

Hope this helps!

Lucia