Time served was a punishment in which all or part of one's sentence was deemed to have already been served prior to sentencing, thereby shortening or negating their subsequent term of punishment.
In 2378, Captain Kathryn Janeway decided to punish The Doctor for his insubordination with a sentence of six days in which he could not use his mobile emitter (thereby confined to quarters), which she then declared time served, as he had not left USS Voyager's sickbay in a week already. (VOY: "Renaissance Man")