If I understand your question, you are trying to get the DST offset for a given date for a given zone.
In the Rule class, there is a property called Rule.Save which is the amount of time to add when this rule is being applied. It returns a TimeSpan. The name is a bit confusing but it was chosen as it matches the column it reads from in the zone database files.
There is a method on the Zone class called Zone.GetRule which will be able to give you the rule object for a given date and zone. Unfortunately this method is private right now but if you make it public and call it with the right parameters you should be
able to get the Rule object out to get the 'Save' information.
Please let me know if I have misunderstood your question.