Dynamic date slicer

Okay, see attached file for a solution: 

Add a Custom Column to your data table using Power Query > Add Column > Custom Column

Duration.Days( [Run in with Cops] - [Earliest Villain Date])

Then create a new measure: 

Run in is within Selected Time Period = 
VAR _DaysWithin = SELECTEDVALUE( 'Time Slicer'[Days] )
VAR _Result = COUNTROWS( FILTER( 'Table', ABS('Table'[Days Since Earliest Villain Date]) <= _DaysWithin) )
RETURN
IF( MAX( 'Table'[Days Since Earliest Villain Date] ) = BLANK(), BLANK(), _Result )

I also added a date table for good practice, but you don't need it yet here: https://excelwithallison.blogspot.com/2020/04/dimdate-what-why-and-how.html