I understand that EC2 Reserved Instances are a billing feature.
I purchase 1 Reserved Instance for an m5.2xlarge in AZa and pay all up-front for the year.
Let's say I run 2 m5.2xlarge instances in AZa for the first 30 minutes fo the hour, then terminate both for the next 30 minutes.
Will I incur any on-demand costs for that hour, or will my single RI cover the EC2 costs for that hour?
According to: Announcing Amazon EC2 per second billing
With Reserved Instances usage you can launch, use, and terminate multiple instances within an hour and get the Reserved Instance Benefit for all of the instances. Bills will now show times in decimal form instead of in minutes and seconds.
So you should not incur any on-demand charges.