Unique value count only at start date

Hi, bdeleur 

You can try the following methods.

Column = 
Var _count= CALCULATE(COUNT('Table'[Reference]),ALLEXCEPT('Table','Table'[Start date]))
Var _mindate=CALCULATE(MIN('Table'[Start date]),ALLEXCEPT('Table','Table'[Reference]))
Return
IF([Start date]=_mindate,_count,BLANK())

Is this the result you expect?

Best Regards,

Community Support Team _Charlotte

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