Define exact time period

[Ksharp]

You mean in the range of +/- 3 month , NOT out of the range ?

data want;
merge have have(where=(b='SCREEN') rename=(visit_date=_date)); 
by id; 
if b eq: 'MONTH' then do;    
if intnx('month',_date,-3,'s') le visit_date le intnx('month',_date,3,'s') then do;
  flag=ifc(visit_date gt _date ,'BELOW','ABOVE'); 
end; 
end;
run; 

Xia Keshan