#  DOW = ([23m/9] + d + 4 + z + [z/4] - [z/100] + [z/400] - 2 (if m>=3)) mod 7
#
#  y is the year.
#  m is the month.
#  d is the day.
#  z = y - 1 (if m <  3)
#    = y     (if m >= 3)
#  A mod B means take the reminder of A / B.
#
#  The source: Journal on Recreational Mathematics, Vol. 22(4), pages 280-282, 1990.
#  The authors: Michael Keith and Tom Craver.
#
#  The formula can be implemented by the following C function:
#
#  int dayofweek(int y,m,d)
#  {
#    return((23*m/9+d+4+(m<3?y--:y-2)+y/4-y/100+y/400)%7);
#  }

sub dow {
  my ($y, $m, $d) = @_;
  $y-- if $m < 3;
  $d += 11 if $y < 1752 || $y == 1752 && $m < 9;
  if ($y >= 1752) {
    return (int(23*$m/9)+$d+4+($m<3?$y+1:$y-2)+int($y/4)-int($y/100)+int($y/400))%7;
  } else {
    return (int(23*$m/9)+$d+5+($m<3?$y+1:$y-2)+int($y/4))%7;
  }
}