You can either set a before event buffer time, an after event buffer time, or both. Here's how to do that:
Select your event type where you wish to add the buffer time
To add a buffer time, click on the Limits tab
Then, click on the before event, or after event select box to view the buffer time options in minutes and select the buffer time of your choice
Click on Update to save your changes. Your event type should now consider the newly added buffer time