Question on Date and URL format in source

Another thing I noticed with this, besides the formatting - when I reference .PublishDate, like this:

    "datePublished": "{{ .PublishDate.Format $.Site.Params.ISO8601 }}",

… but without setting it in YAML frontmatter, it is set to a date far in the past (sorry, can’t remember what exactly).

If that is a built-in, why would it not get the date correct? Or, am I missing something about how this is supposed to be used?