If you've got access to your database, make a user entry in the table serendipity_authors (or with whatever table prefix you created the database).
Just fill in all fields, and afterwards run a SQL script like this:

UPDATE serendipity_authors SET password = MD5('second_authors_pwd') where username ...

The language file contains the parameter (If you use the english language file, this is: serendipity_lang_en.inc.php):

@define('DATE_FORMAT_1', 'l, F jS Y');

Change this into:

@define('DATE_FORMAT_1', 'Y-m-d');

for the format that you asked for (EEYY-MM-DD).

You want your blog in Dutch ?
Find a dutch translation here: http://eurogiant.nl
Instructions are on-site, please leave your comments either here or on my blog.

Wil je de blog in het Nederlands?
Hier vind je een nederlandse vertaling: http://eurogiant.nl
De instructies staan op de site, laat hier ...