I had a problem with this because my server ABSOLUTELY had to have its date in MM/dd/yyyy format, while I wanted the directory to be in YYYY-MM-DD format for neatness sake. Here's how to get it in YYYY-MM-DD format, no matter what your regional settings are set as.
Find out what gets displayed when you use %DATE%:
From a command prompt type:
ECHO %DATE%
Mine came out 03/06/2013 (as in 6th March 2013)
Therefore, to get a directory name as 2013-03-06, code this into your batch file:
SET dirname="%date:~6,4%-%date:~0,2%-%date:~3,2%"
mkdir %dirname%