Date format of the "Authored on" field produces an invalid default value and subsequent form error; saving an Article fails

Problem/Motivation

Noted by dcmistry while setting up a usability testing site:

In this screenshot, the Authored on date field has an invalid default value, resulting in a form error on submission.

Proposed resolution

Determine where this incorrect default value pattern is set.

Remaining tasks

Propose a solution in a patch.