b=mdy(2,29,1981);
if (b=.) then .... not 闰年

if the year is a century year and is NOT divisible by 400 then that century year is NOT A LEAP YEAR.

I find an example online

http://www.sconsig.com/sastips/tip00039.htm         

Try this Macro.

option Symbolgen Mlogic Mprint;
%macro leapyr (year);
data _null_;
lst_dt="31Dec&year"d;
c=substrn(put(juldate7(lst_dt),7.),5,3);
if c ne "366" then put "&year is NOT Leap year.";
else put "&year is Leap year.";
run;
%mend leapyr;

%leapyr(2000);

---------
Results:
2000 is Leap year.
2001 is NOT Leap year.
2002 is NOT Leap year.
2003 is NOT Leap year.
2004 is Leap year.

如果楼主是想找到fyear1那一年endmonth对应的月的最后一天，可以用下面的代码：

data test;
  input fyear1 endmonth  date : yymmdd8.        roa;
  format date yymmdd8.;
  cards;
2000   02            19980203     0.5
2000   05            19980204     0.6
2000   07            19980203     0.5
2000   08            19980204     0.6
2000   11            19980203     0.5
2000   12           19980204     0.6
2100   02            19980203     0.5
2008   02            19980204     0.6
2200   02            19980203     0.5
2400   02            19980204     0.6
;

data want;
   set test;
   if endmonth ne 12 then newvar=put(mdy(endmonth+1,1,fyear1)-1,yymmdd10.);
   else newvar=put(mdy(endmonth,31,fyear1),yymmdd10.);
run;