Ditto the replies above...build date and model year can be different. In the USA market...September is usually considered the "roll-out" month for the new model year...but due to circumstances...sometimes this time frame does not work.
The e39 had a June/1996 roll out date...and it debuted as a 1997 model year...and this was due to the US federal OBD II mandate which the e39 didn't meet the requirement for the usual September debut. Whereas the rest of the world got a 1996 e39...the US market didn't. The e39 debuted in the USA as a 1997 model...and the e34 5 series continued to get 1995 models through March/April 1996. So the US never had an official 1996 5 series model.
Also, the US DOT requires car manufactures to indicated the official model year designation in the 10th digit of the VIN (regardless of build date). See the chart below for how to tell the "official" model year of any car sold in the USA since 1981.
If the 10th VIN digit of the vehicle in question is a
"D"...then it is a
2013 model year (regardless of the 2012 build date). If the 10th VIN digit is a
"C"...then it is officially a
2012 model year: