$ perl -MDateTime -le'my($y,$doy)=(2008,1);my$dt=DateTime->new(year=>$y,month=>1,day=>1);$dt->add(days=>$doy);print$dt->strftime("%Y%m%d")'
20080102
$ perl -MDateTime -le'my($y,$doy)=(2008,1);my$dt=DateTime->new(year=>$y,month=>1,day=>1);$dt->add(days=>$doy-1);print$dt->strftime("%Y%m%d")'
20080101
$ perl -MDate::Calc=Add_Delta_Days -e'my($y,$doy)=(2008,1);printf"%4d%02d%02d\n",Add_Delta_Days($y,1,1,$doy-1)'
20080101