There are legions of cases that make crystal clear that even though the rule of lenity is supposed to be a cardinal principle of statutory interpretation in the criminal context, in practice, it's pretty much meaningless.
Except in this case.
A vice principal at a high school has sexual relations with a 14-year old student. Oops. He pleads no context to a plethora of charges, some of which were alleged to have occurred prior to October 1, 2011, some of which were alleged to have occurred thereafter, and some of which may have occurred in either of these categories.
This matters. Since, depending on the relevant date of his criminal acts, he gets four-for-four custody credits under one set of rules, but only two-for-four under another.
Which adds up to a fair piece of time. Even in the context of his 15-plus year sentence.
The thing is, the statute sets up one rule for crimes before October 1, 2011, and another for crimes after that date. And it's crystal clear that it's the date of the actual crimes that matter, not the date of sentencing or any other date.
The statute also doesn't tell you anything about what you do when there are some crimes before that date and some crimes after. Plus, all the parties agree that there's no "middle ground". It's either one rule or the other. You can't "mix and match" and apply one rule to one part of the sentence and another to the rest.
Hmmm. What to do?
In a very concise opinion, Justice Premo holds that what you do is to rely on the rule of lenity. It's totally unclear. So you adopt the interpretation that favors the defendant. So the guy gets the more favorable good conduct credit regime.
Still stays in prison for a long, long time. But gets out a bit earlier. Thanks to a doctrine that's supposed to matter a lot but actually matters very little.