Did Ferrari change the year designation system?

I took delivery of my new 2011 458 in December 2010.

So, it must have started before that year.

 

Why do you think it changed? Ferrari follows the US federal regulations on vehicle model year designations, just like every other manufacturer who sells cars in the US.

So no, your assumption is incorrect. The regulations require a model year designation vehicle to be produced no earlier than Jan 1 of the year prior to the designated year, and no later than Dec 31 of the designated year - ie, production of that designated model year can only span one Jan 1 date, on the designated model year.

Ferrari could start production of the new GTC4Lusso right now, in Feb 2016, as a 2017 model year vehicle, and produce it right until Dec 2017, as a 2017 model year, under US law. Many manufacturers do early model year introductions for new vehicles, then produce them until the normal changeover in the summer of the following year - for example, the brand new Mazda Miata, introduced in the US as a 1990 model, began production in April 1989, went on sale in July 1989 - but were 1990 models, and were produced through to July 1990. In July 1990, the model year designation switched to the 1991 models, and those 1991 models began showing up in Mazda showrooms in September 1990.

Think back to September 2015 - what were the model years of all the new cars showing up in the various manufacturers' showrooms? 2016 models. Across the entire industry, US models for the following model year designation typically begin production in July or August of the previous year, show up in showrooms around September, then are built until July or August of the model year.

So...

It is normal for 458s built in the second half of 2011 to be 2012 model year cars in the US. Same for every other manufacturer selling cars in the US, for the past 50 years at least.

 

Never been that way in the modern era on US cars. Year model is a function of Federal law. It is a legal indicator of what set of federal and state standards the car was built to comply with and manufacturers get some discretion on it. In the 80's and 90's the start month and end month of a US model year was often different every year and it is really never Jan-Dec. There are also many cases where # 1234 is MY 99 and #1344 is MY 98.

 

In 1983 as an example Ferrari built 83 models from March 83 until December 83. For 84 they started in December 83 and stopped in mid July 84. Then in 85 they went from mid July 84 until mid July 85. Typically they start in September and stop in August but there are almost as many exceptions as not.

 

Fairly common on Euro cars.
If it was built to an '07 build sheet, it's an '07.
Those built to an '08 build sheet are '08s, even if built in '07.

Notwithstanding, they might start using some newer parts early, when they run out the inventory of the old parts.
You see that a few times in the spare parts catalog: "part XXXXX until depleted, then YYYYY".
Or they might even list the serial number when they switched.