Best practice for node expiration with email notification
Drupal has a custom module available called Rules with a submodule Rules Scheduler that will allow you to set up custom scenarios for when you want a certain action to take place. 

In your specific scenario you would use Rules and Rules Scheduler to set up custom emails to be sent out at your specified time based off of your time field. 

Node one has several tutorial videos available to watch with a specific video available about how to use content fields and rules scheduler to set up time specific actions.