Larry, of course that’s the right approach from a PM point of view. What I was looking for was a way to make the plan in MS Project contain this constraint, meaning that if development is delayed beyond Jan 1st 2016, Project will automatically prolong the test period to 2017 to make sure it contains 3 months winter in total. Can this be done by e.g. defining a ‘Winter’-resource, available only in winter time? (I have tried but have very limited experience in working with resources in Project …) Or is there an other way?