Once a day OIT makes backups of the files on disk to magnetic tape. On weekdays this backup is done in the evening, and on weekends it is done during the afternoon. An incremental backup copies any file that has been created or modified since the previous day's backup. Incremental backups are written over once a week, e.g., the Monday incremental backup tape gets written over every Monday. The big incremental backup copies any file that has been created or modified since the previous Wednesday's backup.
Full backups copy everything on disk.
These backups are kept for one year. Files that you keep for a long time but never modify will only be captured on full backups. Generally, the more often you modify a file the more backups it will be captured on. Once you know whether your file was on the system long enough to have been copied by the backup, you can then use fl the file listings program to find the exact backup you want your file restored from. You want to find the most recent backup your file is on.