Same as @ballardw already posted with the addition of the calculation for the base date.
data have;
input col1 col2 $ col3;
datalines;
1391 catA 84
1392 catA 25
1475 76 35
;
data want;
set have;
if _n_=1 then base_dt=intnx('month','01jan2001'd,-1391,'b');
retain base_dt;
format base_dt date9.;
date = intnx('month',base_dt,col1,'b');
format date monyy7.;
run;
proc print data=want;
run;
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