data a;
  input company date:yymmn6. sales;
  format date yymmn6.;
  datalines;
1001 200509 20
1001 200603 30
1016 200603 451
1016 200606 521
1016 200609 358
1016 200612 650  
;
run;
proc sort data=a;
by company date;
run;
data c;
set a;
by company date;
if _n_=1 then date2=date;
else date2=lag(date);
y=intck("month",date2,date );
run;

这里面为什么date2第二行是有缺失值的？请教，谢谢！