I don't understand how your "count manually" column is created. Here is how you can see the months between 09Mar2017 and Feb 2020
data test;
a='09Mar2017'd;
d='01feb2020'd;
a1= INTCK('MONTH',a,d);
do i = 0 to a1;
anniv = intnx('MONTH',a, i, 'same');
output;
end;
format a d anniv DATE9.;
run;
proc print;run;
... View more