If, after reading the other questions and viewing the links mentioned in the comment sections, you still can't figure it out, read on.
First of all, where you're going wrong is the offset.
It should look more like this...
set mydate=%date:~10,4%%date:~6,2%/%date:~4,2%
echo %mydate%
If the date was Tue 12/02/2013 then it would display it as 2013/02/12.
To remove the slashes, the code would look more like
set mydate=%date:~10,4%%date:~7,2%%date:~4,2%
echo %mydate%
which would output 20130212
And a hint for doing it in the future, if mydate equals something like %date:~10,4%%date:~7,2% or the like, you probably forgot a tilde (~).