How exactly does AWS EC2 count hourly costs?

Question

Simple question:

If I had six identical EC2 instances process data for exactly ten minutes and turn off would I be charged six hours or one hour?

Answer 1

AWS has introduced per second billing for EC2/EBS effective October 2, 2017.

New EC2 Billing

Answer 2

Granularity for changes are measure down to the hour.

From the AWS pricing site http://aws.amazon.com/ec2/pricing/:

Pricing is per instance-hour consumed for each instance, from the time an instance is launched until it is terminated or stopped. Each partial instance-hour consumed will be billed as a full hour.

Unless you are calculating time to be under a threshold for a free tier, the second you use an EC2 instance you're charged for the full hour. If you go one second over the first hour, your charged for a full second hour.

One caveat: Spot Instances.

If spot instances are interrupted by AWS (not you) before reaching a full hour use, you're not charged at all. If you interrupt the spot instance, you're charge for the partial hour usage (which is a full hour rounded up as per the on-demand instances).