Upstreaming from https://phabricator.wikimedia.org/T110
/settings/panel/account/ seems to only allow setting a custom Time Format, but not the Date Format (also tested this here in upstream).
Setting a custom format like Y-m-d H:i ends up with a duplicated date as the custom formatting is only appended:
Mon, Jun 9, 2014-06-09 12:08
I'm aware of T1321#12 for server config; however the user setting should be expanded to support not only editing the time format, but also the date format.