The backup file takes the form of:
[backup]-[Month]-[Day]-[Year]-[Backup Number].
If you created several backups on the same day, they would look like:
backup-May-15.2003-1.tar.gz
backup-May-15.2003-2.tar.gz
backup-May-15.2003-3.tar.gz
This feature prevents you from accidentally overwriting a backup file.