I'm buying a BX2380 and the serial number is 18331. Is this a model year 2017 or 2018 tractor?
Contact a dealer, if it's new, it's new, the Serial#'s don't matter for date on tractors, time in service, (date first sold), is the year of the tractor.
In other words, it does not matter when was actually manufactured, like if it was manufactured in 2017 but sold in 2018, it's a 2018.
The 80 series is the newest series, and they normally do not change anything on the tractor in a series through the stretch of manufacturing, so a 2017 will be the same build as the 2018 and so on and so on.
Now there are some exceptions to this, like they have discovered a flaw or fault with a part, then they will change that part but they usually will also provide that part for any older model to bring it up to snuff.