I have since come up with this, but if anyone has better suggestions please let me know;
CALCULATE([Tunits],FILTER(ALL(Table1[DATE]),Table1[DATE] < MAX(Table3[enddate]) && Table1[DATE] > MAX(Table3[sdate]) ))
I first overcoplicated things by using relatedtabe, forgetting that calculate see entire data model so had this ;
btwn:=CALCULATE([Tunits],
FILTER(ALL(Table1[DATE]),Table1[DATE] > MAXX(RELATEDTABLE(Table3),Table3[sdate])
&& Table1[DATE] < MAXX(RELATEDTABLE(Table3),Table3[enddate]) ))
Richard
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