Error message on post editor about Date format strtotime

While I’m editing posts, I get an error message in the Overview section at the bottom of the page which says:

Piwik error: Date format must be: YYYY-MM-DD, or ‘today’ or ‘yesterday’ or any keyword supported by the strtotime function (see https://php.net/strtotime for more information):

It doesn’t seem to have any impact apart from taking up space on the screen, as far as I can tell. But it appears on every post. (It doesn’t appear on Pages).