Unfortunately, legal definitions can have counterintuitive consequences.
The article is saying that the justices disagree on when you become an adult. One justice is saying that you become an adult 18 years after you're born, but how are years defined by the law?
It could be defined as "365 days, or 366 days if the time period starts after February 28th of a year preceding a leap year". Instead, the law first defines "N months" to always end on the last day of the final month if there aren't enough days in that final month, and then a year is simply 12 months.
This is actually sensible. If you enter a 1-year contract on Feb 29th, it makes sense that it ends on Feb 28th rather than Mar 1st the next year.
Therefore you become an adult 216 months after you're born, which in the case of this girl happened on the last day of February 2018 (because February 29th 2018 didn't exist). I agree that this is a problem, but if the problem lies in the law's definition of "becoming adult", it might not even be possible for judges to fix it.
Assuming it is actually possible for them to fix it, the alternative definition could have been "on the day of your 18th birthday", but really you need a more precise the definition actually must accommodate February 29th and push it to March 1st. For example, as the other justice puts it, "until your 18th birthday has begun or passed" (as if February 29th 2018 existed but was of length zero). Of course you'd have to find out whether any other laws are in conflict with this definition, and possibly fix all the computer programs that were assuming the old definition (for example, would she have voted on February 28th 2018? would that change with the new definition?).
BTW at least in Italy, if the justices reach parity they have to judge in favor of the defendant. In that case she would be declared not an adult. But the ancillary question of e.g. elections would not be resolved.