What is a service license?

A service license (Hardware Replacement Option) is dependent on the activation date of an appliance.
The license starts when the appliance is initially activated, exactly like the subscription license.
 
For example:
 
06.2009 - Appliance is activated
12.2009 - A 365 day service license is added to the appliance.
06.2010 - The service license expires, the supposed end 12.2010 is false.