No app required -- all you need is the serial number.
For example: W8733xxxxxx was built in 2007, week 33.
I believe the first 2 characters identify the assembly plant -- with "W8" being (Shanghai?) China.
I don't know the codes, but every iMac I've seen starts with "W8" -- so it's not very interesting.
My 20" ALU (Dud #1): W87313PEX86 = 2007, week 31
My 24" ALU (Dud #2): W873447ZX89 = 2007, week 34
My 20" white upgrade: W8733xxxxxx = 2007, week 33
...LOL, my "older model" white 20" is newer than the ALU 20" dud,
LK