Obtain Unique values within distinct values

Hi kbruce ,

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

1. Create a calculated column to get the Year

Year = YEAR('Table'[Date])

2. Create two measures as below to get the count of new clients

Measure = 
VAR _minyear =
    CALCULATE ( MIN ( 'Table'[Year] ), ALL ( 'Table'[Year] ) )
VAR _curyear =
    SELECTEDVALUE ( 'Table'[Year] )
VAR _preclients =
    CALCULATETABLE (
        DISTINCT ( 'Table'[Client name] ),
        FILTER ( ALL ( 'Table' ), 'Table'[Year] < _curyear )
    )
VAR _curclients =
    CALCULATETABLE (
        DISTINCT ( 'Table'[Client name] ),
        FILTER ( 'Table', 'Table'[Year] = _curyear )
    )
RETURN
    IF (
        _curyear = _minyear,
        DISTINCTCOUNT ( 'Table'[Client name] ),
        COUNTROWS ( EXCEPT ( _curclients, _preclients ) )
    )

Count of new clients = SUMX ( VALUES ( 'Table'[Year] ), [Measure] )

Best Regards