I've looked through the existing documentation on the "Front Matter" and have not been able to find an option to exclude content from appearing in the sitemap.

This use case arises when we want content to be included in "master" pages. For example, our careers page lists all careers by ranging over .Data.Pages and displaying the title and content (which amounts to a brief description), though we technically do not want each individual career to have its own landing page.

Normally this isn't an issue, but it becomes one on the sitemap.xml as the pages, which shouldn't really have their own URL, are included in the sitemap. As these pieces of content don't have dates associated with them, Google Webmaster Tools reports an invalid date. It also produces a fairly pointless URL with negligible content.