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Do you mean with declaratively or with code? The documentation is fairly good on working declaratively, especially the implementation guide (http://resources.docs.salesforce.com/latest/latest/en-us/sfdc/pdf/salesforce_entitlements_implementation_guide.pdf). For working with code, this (https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_interface_support_milestonetriggertimecalculator.htm⌗!) documentation may be a useful starting point.