Why do people specify date as an option for \usepackage?
From source2e.pdf, page 479 (CTAN page or texdoc source2e in a terminal):
![<code>\usepackage[<package-option-list>]{<package-list>}[<version>]</code>//There can be any number of these declarations. All packages in <code><package-list></code> are called with the same options.//Each <code><package></code> file defines new elements (or modifies those defined in the <code><class></code>), and thus extends the range of documents which can be processed. The <code><package-option-list></code> is a list of options which can modify the formatting of elements defined in the <code><package></code> file. The <code><version></code> is a version number, beginning with a date in the format <code>YYYY/MM/DD</code>. If an older version of the package is found, a warning is issued.](https://i.stack.imgur.com/1dpIx.png)
Phelype Oleinik pointed you at the LaTeX sources but there is also a user guide called usrguide.pdf
texdoc usrguide
will open the document in texlive. Section 3.2 on preamble commands has the syntax for \usepackage.
This argument was extended with new syntax last year so you may also want to look at LaTeX News 28 in texdoc ltnews which points you at this article from TUGBoat on the project website
https://www.latex-project.org/publications/2018-FMi-TUB-tb122mitt-version-rollback.pdf