Effective October 2nd, usage of Linux instances that are launched in On-Demand, Reserved, and Spot form will be billed in one-second increments. Similarly, provisioned storage for EBS volumes will be billed in one-second increments.
This change is effective in all AWS Regions and will be effective October 2, for all Linux instances that are newly launched or already running. Per-second billing is not currently applicable to instances running Microsoft Windows or Linux distributions that have a separate hourly charge. There is a 1 minute minimum charge per-instance.
List prices and Spot Market prices are still listed on a per-hour basis, but bills are calculated down to the second, as is Reserved Instance usage (you can launch, use, and terminate multiple instances within an hour and get the Reserved Instance Benefit for all of the instances). Also, bills will show times in decimal form.
The Dedicated Per Region Fee, EBS Snapshots, and products in AWS Marketplace are still billed on an hourly basis.