It seems like since the check above says `System.currentTimeMillis > nextRolloverTime` (i.e. it's strictly greater) I'm having trouble constructing a case where this `+ 1` is necessary. What is the case?

If `shouldRollover` was not strict inequality, I could see a case where, in the same millisecond, you finished one interval and rolled over to another and then just re-calculated the same one. But that doesn't seem to be the case here.