The @date directive assigns a date to a page. If a date is specified it will appear at the top of the webpage (if there is any content on the page).
For example the Florida 2001 page has at the top:
@title Florida @date Sunday, 24 June 2001
specifying its date as 24th June 2001. Just to get the date appearing on the page it could have been embedded into the markdown of course. The @date directive is important for ordering the links to child or siblings on a page.
By default pages are listed in lexicographical order of their filenames. An explicit ordering can be forced by putting a number on the front of the filename. For example you should see the webpage address for this webpage has a 2020_ on the front of it.
If the webpages aren't numbered like that then another ordering can be imposed by giving a date to the webpages. This is particularly aimed at the diary entries to force them to be sorted in date order (which makes a kind of sense).