The restricted single-machine common due date problem can be stated as follows: A set of n jobs with deterministic processing times pi and a common due date d are given. The jobs have to be processed on one machine. For each of the jobs an individual earliness αi and tardiness βi penalty is given, which is incurred, if a job is finished before or after the common due date d, respectively. The goal is to find a schedule for the n jobs which jointly minimizes the sum of earliness and tardiness penalties. Even simple in the formulation, this model leads to an optimization problem that is NP-Hard.