During a Supreme Court argument hour swimming with numbers and studded with a few humorous moments, the justices on Tuesday wrestled with the standard for calculating good-time credits for federal prison sentences.

The issue before the Court in Barber v. Thomas is the interpretation of a “term of imprisonment” under the federal good-time credit statute. The petitioning federal inmates argue that they should be eligible for the statutory 54 days of good-time credit for each year of their entire sentence as originally imposed. The position of the Bureau of Prisons is that the calculation of good-time credit is based only on time actually served by the prisoner.