A mechanism for performing retroactive backups in a computer system is presented. The retroactive backup mechanism employs a backup policy dictating that certain backups are to be performed at set times and for set backup levels, but subject to user-defined selection criteria so that the backups are of a retroactive nature. The selection criteria comprises a retroactive backup date upon which the scheduled backup is made effective. It is selected to correspond to the backup date of a previous backup or save set. Alternatively, the retroactive backup date upon which the scheduled backup is made effective is determined by controlling the amount of data to be backed up. Thus, the selection criteria comprises a maximum size threshold, selected by a user or system administrator as the amount of data that can be backed up in an allotted backup time. The retroactive backup copies to a new save set only data that has not been modified or deleted since the date indicated as the retroactive backup date.