Account settings only allow you to change the time–of–day format; it would be nice and i18n friendly to allow users to customize the way they want to display full dates.
For instance, I would like my dates to display in the ISO 8601 format YYYY-MM-DD, the way I have configured it on Wikimedia wikis.
(Pinging @Qgil so that he can triage this feature request at his leisure.)