\parindent=0pt
  \font\big=cmb10 scaled\magstep3
  \centerline{\big FRIDAY THE THIRTEEN}
  \normalbaselineskip=12pt
  \noindent
  \newcount\dw                         % Gets day of the week
  \newcount\leap                        % Leap year fingaler
  \newcount\m                           % Temp register
  \newcount\y                           % Another temp register
  \newcount\a
  \newcount\b
  \newcount\x
  \newcount\jd
  \newcount\aantal
  \newcount\kalmaand
  \newcount\alfa
  \newcount\eindjaar
  \newcount\tempjaar
  \newcount\firstyear
  \newcount\lastyear
  \newcount\c
  \newcount\d
  \newcount\e
  \newcount\f
  \newcount\z
  \newcount\jda
  \newcount\jdb
  \newcount\difference
  \newcount\jaar
  \newcount\maand
  \newcount\dag
  \newcount\jr
  \newcount\mnd
  \newcount\dg
%
\parskip=1.5cm
%
\def\daydifference
{
\message{date 1: }
\datjul
\jda=\jd
\message{date 2: }
\datjul
\jdb=\jd
\advance\jdb by-\jda
\difference=\jdb
\ifnum\difference<0
\multiply\difference by -1
\fi
\hbox{\number\difference}\hfill\break
}
\def\dayafter
{
\message{date 1: }
\datjul
\message{day difference :}
\read16 to\difference
\advance\jd by\difference
\number\difference\hfill\break
\z=\jd
\juldat
\hbox{\number\jr\ \number\mnd\ \number\dg}\hfill\break
}

\def\datjul{
%
%
%
%
\jaar=1991
%\read16 to\maand
%\read16 to\dag
%\hbox{\number\jaar\ \number\maand\ \number\dag}\hfill\break
\global\eindjaar=\jaar
\global\advance\eindjaar by15
\maand=1
\dag=1
\ifnum\maand>2
   \y=\jaar
   \m=\maand
\else
   \y=\jaar \advance\y by-1
   \m=\maand \advance\m by12
\fi
\a=\y
\divide\a by100
\b=2
\advance\b by-\a
\x=\a
\divide\x by4
\advance\b by\x
%
\jd=\y
\multiply\jd by36525
\divide\jd by100
%
\x=\m
\advance\x by1
\multiply\x by306001
\divide\x by10000
\advance\jd by\x
\advance\jd by\dag
\advance\jd by1720995
\advance\jd by\b
}
%
\def\dayofweek{
\x=\jd
\advance\x by 1
\dw=\x   \divide\x by 7 \multiply\x by 7 \advance\dw by -\x
}

%
%
\def\juldat{
\ifnum\z<2299161
   \a=\z
\else
   \alfa=\z
   \multiply\alfa by100
   \advance\alfa by-186721625
   \divide\alfa by3652425
   \a=\z
   \advance\a by1
   \advance\a by\alfa
   \divide\alfa by4
   \advance\a by-\alfa
\fi
%
\b=\a
\advance\b by1524
%
\c=\b
\multiply\c by100
\advance\c by-12210
\divide\c by36525
%
\d=\c
\multiply\d by36525
\divide\d by100
%
\e=\b
\advance\e by-\d
\multiply\e by10000
\divide\e by306001
%
%
\x=\b
\advance\x by-\d
\dg=\x
\x=\e
\multiply\x by306001
\divide\x by10000
\advance\dg by-\x
%
\mnd=\e
\advance\mnd by-1
\ifnum\e>13
    \advance\mnd by-12
\fi
%
\jr=\c
\advance\jr by-4715
\ifnum\mnd>2
   \advance\jr by-1
\fi
}
%
\def\leesjul{
\relax
\read16 to \z
}
\newdimen\cf
\setbox0=\hbox{\rm0}
\cf=\wd0
\def\breedte{\kern\cf}
\def\dertien{
\datjul
\kalmaand=\maand
\dayofweek
\z=\jd
\juldat
\loop\ifnum\jr<\eindjaar
   \dayofweek
   \ifnum\dg=13
       \ifnum\dw=5
       \hbox{\number\jr\ \number\dg\ \number\mnd}
       \fi
   \fi
   \advance\z by1
   \jd=\z
   \juldat
\repeat
}
%
\dertien
\bye