Hi Henrik,
If you purchased this battery, then the manufacture date is likely correct and cannot be changed. These are OEM batteries, made by Apple, around 2012-2013. If you are encountering any isses with the battery you received, we recommend contacting our Support teams directly:
iFixit US: http://help.ifixit.com/collection/130-contact-us
iFixit Europe: http://ifixit-eu-de.helpscoutdocs.com/
I hope this helps!