Service Level Agreements (SLAs) are very common in the support industry. Salesforce makes these very easy to apply and manage via the built in Entitlement Processes. These entitlement processes will generate Case Milestones that are associated with your case. These milestones will have a Target Date that shows you when your Case Milestones have to be completed. Typically the date itself is fairly difficult to use however if you convert this into the number of minutes remaining then you get a tangible count down for an action that has to be done on the case. This number is called your Service Breach Time (SBT).
Unfortunately there is no easy way to transfer the Target Date from your Case Milestones to your Case and generate your SBT. These fields cannot be pulled into a formula and you cannot write triggers against Case Milestones. You can however use Apex to get the data onto your Case object.