Date of assembly is stated as date of purchase

The date starts when you purchased the MBP not when you received it. If not happy with that, you still can return it back Apple and or contact Apple Support.

Get Support – Apple

https://getsupport.apple.com/

Call Customer Support (800) MY–APPLE (800–692–7753)

or online https://getsupport.apple.com/

or call AppleCare Support at 1-800-APLCARE (800-275-2273)

Outside the USA—Contact Apple for support and service by phone

See a list of Apple phone numbers around the world.

https://support.apple.com/en-us/HT201232

If the date is incorrect, then you just need to present Apple with a copy of the purchase receipt to correct the issue either at the time of a repair if you actually need a warranty repair on those disputed days or you can do so now by having Apple update their records. Depending where the laptop was purchased Apple may not have been given your purchase information (or perhaps has not processed it yet to update the online information) so Apple ends up using the date of manufacturer unless the owner shows an actual receipt for proof of purchase.

I would get on the phone with Apple Support and see what they can do to assist you in sorting out this issue.