Harvest does not have a specific feature for tracking overtime. As a workaround for tracking overtime, you’ll want to create a task called “Overtime” (or something similar—name it whatever works for you) that you can then assign to your projects.

If you charge overtime work at a higher rate than normal, be sure to update the overtime task’s billable rate either in Manage > Tasks or on each individual project. Note that you'll only be able to bill a different amount for your overtime task if the project bills by Task billable rate. 

It is not possible for Harvest to automatically detect when someone is working overtime and switch to that task—people will need to manually begin tracking time to that task when appropriate.