Peter writes:

Justice Kennedy noted that the total difference, when you count all the years of all the sentences, is 36,000 years of illegal time served (p. 53). And Justice Stevens noted that it might involve $100 million in wasted money (illegally spent) on keeping people locked up an extra week per year each (p. 42).

CNN said the justices had a merry time with the Barber case:

The justices managed to crack themselves up -- along with the public audience -- at least a dozen times in the hourlong oral debate. Justice Clarence Thomas rarely speaks at the high court's normally sober sessions, but he especially enjoyed the gentle insults and self-deprecating jibes his colleagues showered on each other. His booming laugh could be clearly heard at times.

...Despite the fun, the question is a serious one for the nearly 197,000 federal prisoners and their families, according to the most recent weekly population report issued by the Justice Department. Ninety-five percent of the inmates are affected by the good-time provision. Lawyers for the inmates say the savings to taxpayers under their reading of the law would amount to $953 million a year now being spent to incarcerate current prisoners.