Based on the comments.
The lambda function would be only required to start your instance, if you want to trigger it using CloudWatch (CW) Event rule. The reason is, that you can stop the instance directly, using CW built-in targets:
Built-in targets: EC2 CreateSnapshot API call, EC2 RebootInstances API call, EC2 StopInstances API call, and EC2 TerminateInstances API call.
Sadly, there is no build-in target for starting the instance, thus lambda function is needed for that.
CLICK HERE to find out more related problems solutions.