Apply by doing:
       cd /usr/src
       patch -p0 < 013_addusery2k.patch
       cd usr.sbin/adduser
       make
       make install

Index: usr.sbin/adduser/adduser.perl
===================================================================
RCS file: /cvs/src/usr.sbin/adduser/adduser.perl,v
retrieving revision 1.19
retrieving revision 1.20
diff -u -r1.19 -r1.20
--- usr.sbin/adduser/adduser.perl       1999/09/06 16:48:41     1.19
+++ usr.sbin/adduser/adduser.perl       2000/01/03 19:30:00     1.20
@@ -1057,9 +1057,10 @@
    return 1 if $logfile eq "no";

    local($sec, $min, $hour, $mday, $mon, $year) = localtime;
+    $year += 1900;
    $mon++;

-    foreach $e ('sec', 'min', 'hour', 'mday', 'mon', 'year') {
+    foreach $e ('sec', 'min', 'hour', 'mday', 'mon') {
       # '7' -> '07'
       eval "\$$e = 0 . \$$e" if (eval "\$$e" < 10);
    }