Your battery will be new.
Battery and Mac don't have the same manufacturing date. The Mac's manufacturing date is the assembly date. The battery was made before that.
The Mac's manufacturing date is encoded in the serial number. On a MBP it will start with something like W88xy... where W8 is the factory code for the factory that assembles the MBP in Shanghai, 8 stands for the manufacturing year 2008, and xy is the week of the year 2008 in which it was assembled.
More information can be gathered from your s/n:
Klantenservice: Serienummers