function [j]=mjulian(y,m,d,h,force_true_julian)
%JULIAN  Converts Gregorian calendar dates to corresponding
%      modified Julian day numbers.  
%  
%  Although the formal definition
%      holds that Julian days start and end at noon, here
%      Julian days start and end at midnight.
%
%    In this convention, "true" Julian day 2440000 began at 0000 hours, May 23, 1968.
%                  and modified Julian day   40000 began at 0000 hours, May 23, 1968.
%
%  note that passing dates prior to Nov 17, 1858 will likely generate many
%  unreasonable errors
%                  
%     Usage: [j]=julian(y,m,d,h)  or  [j]=julian([y m d hour min sec])
%
%      OR (as of 15 May 2006, bfleming)
%    
%     [j]=julian(y,m,d,h,force_true_julian)
%  
%     OR
%  
%     [j]= [j]=julian([y m d hour min sec],force_true_julian)
%  
%  so in summary:
%    nargs=1: [y,m,d,hr,mn,sc] --assume modified julian day wanted
%    nargs=2: [y,m,d,hr,mn,sc],force__true_julian --true julian day wanted
%    nargs=4: y,m,d,hr assume modified julian day wanted
%    nargs=5: y,m,d,hr,force_true_julian --true julian day returned
%     ************************************************************
%
%        d.... day (1-31) component of Gregorian date
%        m.... month (1-12) component
%        y.... year (e.g., 1979) component
%        j.... decimal Julian day number
%        h.... decimal hours (assumed 0 if absent)
%        force_true_julian (any value-not just any non-zero value-passed will do)
%
%     ************************************************************
%     recoded for MATLAB  by Rich Signell, 5-15-91
%

%   some alteration to allow for modified julian dates rather than 
%   the (larger) julian date, RJF 5/15/2005
%
%   NOTE: should really reduce complexity and restrict input to 
%   one or two args, array + optional "force_true_julian".


      true_julian=0;
      switch nargin
       case {1,2};
          h=hms2h(y(:,4),y(:,5),y(:,6));
          d=y(:,3);
          m=y(:,2);
          y=y(:,1);
          if nargin==2
            true_julian=1;
          end;
       case 3; 
          h=0.;
       case 5;
          true_julian=1;
      end
      mo=m+9;
      yr=y-1;
      i=(m>2);
      mo(i)=m(i)-3;
      yr(i)=y(i); 
      c = floor(yr/100);
      yr = yr - c*100;
      j = floor((146097*c)/4) + floor((1461*yr)/4) + ...
           floor((153*mo +2)/5) +d +1721119;

%     If you want julian days to start and end at noon, 
%     replace the following line with:
%     j=j+(h-12)/24;
 
      j=j+h/24;
      %oldj=j; 
      if ~true_julian;
       j=j-2400001;
       % or if starting at noon,
       %j=j-2400000.5;
      end;
      %save ~/julstate;