Conditional Measure using slicer selection

Hi Rob92 ,

I created a sample pbix file(see the attachment), please check if that is what you want.

1. Create a sperated date dimension table(DO NOT create any relationship with your fact table) just as suggested by Ibendlin

2. Create two measures as below

Old clients count = 
VAR _mindate =
    MIN ( 'Date'[Date] )
RETURN
    CALCULATE (
        SUM ( 'Table'[Transactions] ),
        FILTER ( 'Table', 'Table'[Client_joined] <= _mindate )
    )

New clients count = 
VAR _mindate =
    MIN ( 'Date'[Date] )
RETURN
    CALCULATE (
        SUM ( 'Table'[Transactions] ),
        FILTER ( 'Table', 'Table'[Client_joined] > _mindate )
    )

3. Create a table visual as shown in the below screenshot

Best Regards

Awesome, thanks guys!  I have implemented the solution on my real data and it works a treat.  The only issue was that as well as the slicer controlling which users are new or old, I also need it to filter the visual.  In order to do this I have joined the date dimension table to the fact table.  This causes the date hierachy to break but that's not a problem - to resolve that, I added separate month and year columns in power query, and used those on the X axis instead.  Problem solved!