Amazon AWS  Chief Evangelist for Web Services Jeff Barr announced on September 18, 2017, that effective October 2, 2017, all Regions of Linux instances will be billed by the second instead of by the hour.

This move competitively places Amazon among cloud providers.

Many instances just require short interval sprints, so the bill by the used seconds is a great resource.

There is a minimum charge of 1 minute per instance, and not available on  Microsoft Windows, which still is billed by the hour.

More information here:

https://aws.amazon.com/blogs/aws/new-per-second-billing-for-ec2-instances-and-ebs-volumes/