https://github.com/aws-samples/aws-blog-firehose-custom-prefixes-for-s3-objects
Destination configuration default
Intermediate S3 prefix
(Optional) To use the default prefix for Amazon S3 objects, leave this option blank. Kinesis Data Firehose automatically uses a prefix in “YYYY/MM/dd/HH” UTC time format for delivered Amazon S3 objects. You can add to the start of this prefix. For more information, see Amazon S3 Object Name Format.
Custom Prefixes for Amazon S3 Objects
When evaluating timestamps, Kinesis Data Firehose uses the approximate arrival timestamp of the oldest record that’s contained in the Amazon S3 object being written.
The timestamp is always in UTC.