function dn = easter(y)
% EASTER  Date of Easter.
% EASTER(y) is the datenum of Easter in year y.
% Ref: Donald Knuth, The Art of Computer Programming,
%      Fundamental Algorithms, pp. 155-156.

%   Copyright 2014 Cleve Moler
%   Copyright 2014 The MathWorks, Inc.
% Golden number in 19-year Metonic cycle.
g = mod(y,19) + 1;

% Century number.
c = floor(y/100) + 1;

% Corrections for leap years and moon's orbit.
x = floor(3*c/4) - 12;
z = floor((8*c+5)/25) - 5;

% Epact.
e = mod(11*g+20+z-x,30);
if (e==25 && g>11 || e==24), e = e + 1; end

% Full moon.
n = 44 - e;
if n < 21, n = n + 30; end

% Find a Sunday.
d = floor(5*y/4) - x - 10;

% Easter is a Sunday in March or April.
d = n + 7 - mod(d+n,7);
dn = datenum(y,3,d);