When a new SLA Definition is created the SLA Timeline can be used to see how the SLA will behave against existing task records.

From the SLA Definition form, open the SLA Timeline by clicking the Validate SLA Definition related link. Select a task by using the task picker in the upper left. The SLA Definition automatically populates for you. When a task is selected that matches your SLA Definition, the SLA Timeline allows you to inspect the different elements of the SLA.