Instance hours are just rounded up.
Source: I auto scale my companies entire stack.
"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."
https://aws.amazon.com/ec2/pricing/
"You are billed for an EC2 instance-hour for each hour or partial hour (rounded up) that your instance is in the “running” state. Instances that are in any other state (“stopped”, “pending”, etc.) are not billed."
https://aws.amazon.com/premiumsupport/knowledge-center/ec2-i...
Here's a company that got hit hard by that behavior :
"A little-discussed fact about AWS EC2 pricing is that users are billed for each server that runs for any partial hour it runs. That means if a user starts a server and then kills it within five minutes, he is still billed for the full hour. That seems acceptable, but if a user kills a server and replaces it with a new server of the exact same type and location, this move doubles the bill."
http://searchaws.techtarget.com/tip/Paying-the-price-when-an...
You should probably take a look at this, you are probably costing your company a lot of money.
If you start and stop one instance 60 time in an hour, you will be charged 60 hours.
So scaling down on a dime and scalling right back up is more expensive than doing nothing.