hey Rocco,

you should be able to do something like this:
 

Basically within a BY you can add/subtract items. Works the same per year.

It would probably be cleaner to do a double time dimension, to show how deferred loss can be accumulated over the future years and from which year that loss comes from

Thank you for the answer, 
It is a great answer:)

And I should note, it works the same for every dimension in Pigment, they are all ordered by default so a + or - works on all of them.