function jdate = julian (month, day, year)

% Julian date

% Input

%  month = calendar month [1 - 12]
%  day   = calendar day [1 - 31]
%  year  = calendar year [yyyy]

% Output

%  jdate = Julian date

% special notes

%  (1) calendar year must include all digits

%  (2) will report October 5, 1582 to October 14, 1582
%      as invalid calendar dates and stop

% Orbital Mechanics with MATLAB

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

y = year;
m = month;
b = 0;
c = 0;

if (m <= 2)
   y = y - 1;
   m = m + 12;
end

if (y < 0)
   c = -.75;
end

% check for valid calendar date

if (year < 1582)
   % null
elseif (year > 1582)
   a = fix(y / 100);
   b = 2 - a + floor(a / 4);
elseif (month < 10)
   % null
elseif (month > 10)
   a = fix(y / 100);
   b = 2 - a + floor(a / 4);
elseif (day <= 4)
   % null
elseif (day > 14)
   a = fix(y / 100);
   b = 2 - a + floor(a / 4);
else
   clc; home;

   fprintf('\n\n  this is an invalid calendar date!!\n');

   keycheck;

   return;
end

jd = fix(365.25 * y + c) + fix(30.6001 * (m + 1));

jdate = jd + day + b + 1720994.5;

