Look here for advice. 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 in the afternoon. An incremental backup copies any file that has been created or modified since the previous day's backup.
Full backups copy everything on disk.
For example, if you created or modified your file after Monday's backup completed, and you delete the file before Tuesday's backup, it has never been backed up. You should consider making your own backups of files that are very important to you. Or, if the files are not particularly large, files may be backed up onto diskettes by downloading them to a microcomputer and then copying them to diskette.