The pricing documentation for RDS indicates:
Pricing is ... from the time a DB instance is launched until it is stopped or deleted.
For RDS, more specifically:
RDS is billed in one-second increments for database instances and attached storage. Pricing is still listed on a per-hour basis, but bills are now calculated down to the second and show usage in decimal form. There is a 10 minute minimum charge when an instance is created, restored or started.
FYI, for regular EC2 instances:
Each time you start a stopped instance we charge a minimum of one minute for usage. After one minute, we charge only for the seconds you use.
For Stopped instances you are charged for any storage being used, but not for the instance itself. Remember that Stopped instances automatically re-start after 7 days.
I believe that no charges accrue when an instance is in "Starting" or "Stopping" state, but my memory is a little fuzzy on that.
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