DATEDIFF function with Filter

What are you doing with FIRSTDATE()? That is a time intelligence function.

This measure will functionally work, but I am not sure it will give you the results are are wanting. You should not use time intelligence functions to do date math. If you are using it intentionally knowing that it invokes context transition, that is fine, otherwise, use MIN() instead, or post back with more info.

Some Measure =
VAR varStartDate =
    TODAY()
VAR varEndDate =
    FIRSTDATE( 'OpportunityBase'[Close By] )
RETURN
    IF(
        MAX( OpportunityBase[StatusCode] ) = 100000023,
        DATEDIFF( varStartDate, varEndDate, DAY )
    )

UCHKANA 

Try the following to see if it works:

Measure Name = 
VAR _max = MAX( OpportunityBase[StatusCode] )
VAR _date = FIRSTNONBLANK( 'OpportunityBase'[Close By] , 1)
RETURN
   IF ( _max = 100000023 ,  
        DATEDIFF( TODAY(), _date , DAY ) ,
        BLANK() 
     )