would just use the dates given as a guide as to whether its early or late flowering but they should start flowering sooner the further south you are as nitepharmer mentioned, however I would not expect it to finish much quicker 2-3 weeks at most for the plant to reach a similiar stage of development.
as per when you harvest it is more personal preference and you could easily harvest 4-5 weeks earlier or let the buds mature longer and harvest only one week earlier than the given finish date.