Any post can define a custom slug, either relative or absolute, which means posts can appear to be "pages" that live outside this date structure. Internally, they still live on disk at their date-based URL but are served from the custom URL instead. For example, you could create a post that contains a resume and make it visible at the following URL:
If the post defines a relative slug, then the full post URL includes that slug. Visiting the unique path component (year/month/day/index) without the slug, or with a different slug, will send a permanent redirect to the new slug.