How to add a Date Picker?


I am new to Yii2 and would like to know if you could help me please.

I am really confused about the way the Yii2 documentation is built.

Here is an example, I want a simple Date Picker, as a beginner my first question is which Class shall I use?

Here is the date picker:

But I cannot find the info on what "use base/class" code I should add.

Could someone please explain to me how shall I read the doc? As I am totally confused.

Thank you,