Hello,
you can choose the format with the parameter $date_format in config.php.
yyyy-mm-dd is not supported as a built-in display format but it is the format used to communicate to the DBMS so the fact you use it for your db is not a problem.
If you want to use it also as a display format, you can build your own custom formatting function or "hack" the format_date function in include/general_functions.php, adding a
[pre]
return $date;
[/pre]
as first line of code.
Best,