The date tag's behavior is somewhat dependent on the context in which it is being called. Per your example, it will either return the published_at date of the current page OR the date of a database item.

If you just want to get the CURRENT date based on the calendar, add the "for" attribute with the value of "now":

<w:date format="%Y" for="now" />

If using the "now" value, remember if the page is cached, it might show the wrong time/date until the next time the page is re-rendered. This is probably not an issue for the "year", though!