Calculate a date with filters

[HadBen]

I'm at a loss. I've been searching for a few days but nothing works. 

I call a series of files, one for every work week, and there's a column with dates that keep changing. I want to see the difference of dates in days between these changes.

I solved this earlier by giving an n and n-1 columns and then merging, with a unique tag and the n=n-1, but this makes the large and slow database impossible to handle. I'd like to do something like this, where I calculate what the date is, but the release is one earlier release. 

Our workweeks are in this format: 2019.01, 2019.02 and so on.

Thank you. 

CALCULATE(MAX(table[Date])
,FILTER('Table','Table'[Unique Tag]='Table'[Unique Tag])
,FILTER('Table','Table'[workweek]='Table'[workweek]-0.01))