Filter (ALL ( Table , Vs. , Filter (ALL( Table Column

[dicken]

Hi,
Can someone help with this or point me in the direction of more information, I've go a measure to give an index based on date, duplicate dates in source,

So I have this:

Index ALL Table:=VAR maxdate = MAX(Table1[Date]) 
            RETURN
        CALCULATE(DISTINCTCOUNT(Table1[Date]), FILTER(ALL(Table1),Table1[Date]<=maxdate) )

Which works, but if I replace FILTER(ALL ( Table1 , with FILTER ALL( Table1[Date] , which is what I tried initially I just get a 1 for each row of the pivot table? I usually only have to refer to the entire table when doing a calculated column.
I've have replaced ALL with ALLSELECTED as it then stays correct when a slicer is used.

Can someone explain what's happening?

Richard