As from Kristjan said.
You've gotta open the Terminal. Then cd to the directory
you want then enter this command.
> find . -exec SetFile -m <date> {} \;
The find command will find all find in specified directory
( in this case . [current] directory ). Then it will execute
command you specified after -exec and {} will be replaced
by file name it found. Last \; will tell it where the command ends.
Cheers, UNIX.