Wow, that's silly. An entire app you'd use only once to find out what week your system was built? Why would I download an entire app just to do this?
Just look at your serial number and look at the 4th and 5th character. Mine says 4H6
232xxxxx, where mine was make in the 23rd week. That's the easy way.
If you need more info, then
http://www.chipmunk.nl/klantenservice/applemodel.html