preferredBackupWindow  
  The daily time range during which automated backups are created if automated backups are enabled, using the BackupRetentionPeriod parameter.
The default is a 30-minute window selected at random from an 8-hour block of time for each Amazon Web Services Region.
Constraints:
- Must be in the format - hh24:mi-hh24:mi.
- Must be in Universal Coordinated Time (UTC). 
- Must not conflict with the preferred maintenance window. 
- Must be at least 30 minutes.