How to add EXIF date from file-date

I have a huge amount JPG photos WITHOUT EXIF date indo.
But it seens like the PS3 only uses EXIF date for sorting.

Så all images without EXIF date ends up in the current
year and month (like "FEB 2008" or "2008") when sorting.

I have the original date in the files "date modified",
but need a program then can create the EXIF date info
from the file-date. But I can only find the opposite :-(

Anyone know how to copy the file-date to the EXIF date
for several >1000 JPG files in an easy way?  Please?
-- 
 Lars-Erik  -  http://www.osterud.name
 -  ICQ 7297605