%%% leap year?
\newif\if@leapyear\@leapyearfalse
\newcount\tmptmp@a\newcount\tmptmp@b
\tmptmp@a=\the@yearcode\tmptmp@b=\the@yearcode
%% if ( year mod 4 == 0 ) then leap year true
\divide\tmptmp@a4
\multiply\tmptmp@a4
\advance\tmptmp@b-\tmptmp@a
\ifnum\tmptmp@b=0
 \@leapyeartrue
\else
 \@leapyearfalse
\fi
\if@leapyear
%% if ( year mod 4 == 0 ) and ( year mod 100 == 0 ) then leap year false
  \tmptmp@a=\the@yearcode\tmptmp@b=\the@yearcode
  \divide\tmptmp@a100
  \multiply\tmptmp@a100
  \advance\tmptmp@b-\tmptmp@a
  \ifnum\tmptmp@b=0
    \@leapyearfalse
  \fi
%% but ( year mod 4 == 0 ) and ( year mod 100 == 0 ) and ( year mod 400 == 0 )
%% then leap year true
  \tmptmp@a=\the@yearcode\tmptmp@b=\the@yearcode
  \divide\tmptmp@a400
  \multiply\tmptmp@a400
  \advance\tmptmp@b-\tmptmp@a
  \ifnum\tmptmp@b=0
    \@leapyeartrue
  \fi
\fi