Calculate repeat values from within user defined periods

I have a  table which looks like

ClientID	Date	Repeats
1	01/04/2018	3
2	02/04/2018	1
3	03/04/2018	1
4	04/04/2018	2
1	04/07/2017	3
6	05/07/2017	1
7	06/07/2017	2
7	07/07/2017	2
5	06/06/2017	1
1	07/06/2017	3
4	08/06/2017	2

Where repeats is a calcuated column:

RepeatCount = 
VAR ClientID = table[ClientID]
return CALCULATE(
    countrows(table),
    ALL(table),
    table[ClientID]= ClientId)

This works well to explain repeats across the whole time period and we have also been able to manage filters to constrain to a particular data range to (e.g. 2018 only) on demand, as required. 

However, user requirement has changed and there is a request to filter the repeats based on a user defined period. 

I'm aware that I can't use slicers to control the calculation, so we clearly need to rethink the calculation. 

Currently, I think if I could add an additional column to show the previous date for that Client then I could use a date reference table to control the slicer against those two colums...

So, powerBi gurus, a couple of questions:

- I'm lost on how to create that additional column - any ideas?

- I'm convinced this is not the most processor efficient/elegant way of achieving this - any broader thoughts on the problem?

thank you!!