Hi Harold, hope you have solved the issue.
Here is my thought, using sample data
Find out customers only in 2016 markdown period, vs rest period customers (2016 + 2017 + 2018, etc)
Create a true/false calculated field.
If True then '2016 markdown customer' (I use superstore data so the logic I keep simple, say purchased in 2016 and Feb - see the screen capture calculated field)
If False then 'non-2016 markdown customer'
From below means 'Allen Armold' has purchased during 2016 markdown (you could use the True/False to narrow down the result)
Much hope if I did not mess up your questions. Cheers.