The en_us language pack does not contain the string for 'strftimedatetimeshort' which typically contains the date format for the language.
This is a particular problem as when developing for a particular language in this case US english the correct way to format a date is to use the langconfig.php datetime format.
The US uses the date format mm/dd/YYYY as opposed to the european / australian english format dd/mm/YYYY the fact that the langconfig.php does not contain the 'strftimedatetimeshort' language string means using this format in the userdate() function returns the european format.
I notice from searching for this issue in the moodle.org forums that this issue has come up in a number of related posts.
I have attached the ammended langconfig.php file to this ticket. Hopefully someone responsible for the en_us language pack will be able to update the en_us language pack and make it available on general release.