Calculate time difference based on report filter

Hi all

I am struggling to work out how to create a measure that will do the following.  I have 2 columns - ArrivalDateTime and DepartureDateTime.  I need to calculate the difference between the two, based on a departure date window (typically a week).  If the arrival was before that week window, I don't want to calculate any time prior to the beginning of the window, so in the example below, using a departure time window of between 24/07/2017 00:00:00 and 01/08/2017 00:00:00, the Duration column is what I'm currently calculating whereas the "Duration I want" is what I hope to calculate (both calculated in days).

I knwo the syntax would be something like:

DepartureDateTime - IF(ArrivalDateTime<MIN(WindowDate),MIN(WindowDate),ArrivalDateTime)

But I don't know how to get MIN(WindowDate)?  It should come from the report filter that uses relative date filtering on DepartureDateTime

For info I'm using a SSAS 2014 Tabular model as my datasource.  Any help would be much appreciated.