Amazon RDS instance backup window duration?

from Working With Backups documentaion

Below is the answer for "what if the backup did not fit into a backup window?"

If the backup requires more time than allotted to the backup window, the backup continues after the window ends, until it finishes.

Below is the answer for "If backup may not fit into the backup window, why do we need a backup window?"

The backup window can't overlap with the weekly maintenance window for the DB instance.

Backup window doesn't specifically ask for the time when to start take backup but instead ask for time period in which aws can trigger backup. So basically it's asking for backup window time. That's why it has 2 fields 1. StartTime: When can the process of backup be started. 2. Duration: time window in which process must start to take backup.

E.G

if I set start time: 5:30 and duration: 30mints

Backup can start at anytime between 5:30 to 6:00.

It's a time window. The backup will take place at some point in time in that window. It might start at exactly the "start time" but it may occur any time after the "start time" as defined by the "Duration".