Since 2000, BSBI date classes are decadal, while Atlas 2020 uses the 20 year period 2000-2019 as its current date class. The DDb provides an option within the subdivide by drop down list to split records before 2000 and since 2000. If we follow the instructions for creating a species list for a vice county subdivided by dates, all we need to do is amend the subdivide by date class field to be pre/post 2000.
So in the example above, based on West Mayo, Abies grandis has been recorded in 2 hectads, 1 hectad pre 2000 and 1 hectad post 2000.