[E90Fleet]

With both German made and South African made BMW's ive tracked (and thats many cars) on my dealers system and compared it to the VIN date its the day the car body goes into Production.
System calls it "Production Start".

It then goes to "vehicle paint" and then "Final line", which is the fitting of all the components and drivetrain before finally being shown as "production end"

The difference between "Production Start" and "Production end" can be 2 days or even a week.
It generally depends what type of a buffer there is between the bodyshop, paint shop and Final line, or if there is a parts supply problem for some reason causing a production sequence shift.
Some lines stop over the weekend, causing a couple extra days between start and end.

At the 3 Series factory here they keep a 24 hour buffer between paint and final assembly, so if there is a problem in paint it does not affect final assembly unless it lasts longer than 24 hours.

Paint is generally just a few hours after start of body production in the body shop.