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+## -*- texinfo -*-
+## @deftypefn {Function File} {} datenum(Y, M, D [, h , m [, s]])
+## @deftypefnx {Function File} {} datenum('date' [, P])
+## Returns the specified local time as a day number, with Jan 1, 0000
+## being day 1. By this reckoning, Jan 1, 1970 is day number 719529.  
+## The fractional portion, corresponds to the portion of the specified day.
+##
+## Years can be negative and/or fractional.
+## Months below 1 are considered to be January.
+## Days of the month start at 1.
+## Days beyond the end of the month go into subsequent months.
+## Days before the beginning of the month go to the previous month.
+## Days can be fractional.
+##
+## WARNING: this function does not attempt to handle Julian
+## calendars so dates before Octave 15, 1582 are wrong by as much
+## as eleven days.  Also be aware that only Roman Catholic countries
+## adopted the calendar in 1582.  It took until 1924 for it to be 
+## adopted everywhere.  See the Wikipedia entry on the Gregorian 
+## calendar for more details.
+##
+## WARNING: leap seconds are ignored.  A table of leap seconds
+## is available on the Wikipedia entry for leap seconds.
+## @seealso{date, clock, now, datestr, datevec, calendar, weekday}
+## @end deftypefn
+
+## Algorithm: Peter Baum (http://vsg.cape.com/~pbaum/date/date0.htm)
+## Author: Paul Kienzle
+## This program is granted to the public domain.
+
+function [days,secs] = datenum(Y,M,D,h,m,s)
+  ## Days until start of month assuming year starts March 1.
+  persistent monthstart = [306;337;0;31;61;92;122;153;184;214;245;275];
+
+  if nargin == 0 || (nargin > 2  && ischar(Y)) || nargin > 6
+    usage("n=datenum('date' [, P]) or n=datenum(Y, M, D [, h, m [, s]])");
+  endif
+  if ischar(Y)
+    if nargin < 2, M=[]; endif
+    error('string form of dates not yet supported');
+    ## [Y,M,D,h,m,s] = datevec(Y,M);
+  else
+    if nargin < 6, s = 0; endif
+    if nargin < 5, m = 0; endif
+    if nargin < 4, h = 0; endif
+    if nargin == 1
+      nc = columns(Y);
+      if nc > 6 || nc < 3,
+        error("expected date vector containing [Y,M,D,h,m,s]");
+      endif
+      s=m=h = 0;
+      if nc >= 6, s = Y(:,6); endif
+      if nc >= 5, m = Y(:,5); endif
+      if nc >= 4, h = Y(:,4); endif
+      D = Y(:,3);
+      M = Y(:,2);
+      Y = Y(:,1);
+    endif 
+  endif
+
+  M(M<1) = 1; ## For compatibility.  Otherwise allow negative months.
+
+  ## Set start of year to March by moving Jan. and Feb. to previous year.
+  ## Correct for months > 12 by moving to subsequent years.
+  Y += fix((M-14)/12);
+
+  ## Lookup number of days since start of the current year.
+  D += monthstart (mod (M-1,12) + 1) + 60;
+
+  ## Add number of days to the start of the current year. Correct
+  ## for leap year every 4 years except centuries not divisible by 400.
+  D += 365*Y+floor(Y/4)-floor(Y/100)+floor(Y/400);
+
+  ## Add fraction representing current second of the day.
+  days = D + (h+(m+s/60)/60)/24;
+
+  ## Output seconds if asked so that etime can be more accurate
+  secs = 86400*D + h*3600 + m*60 + s;
+
+endfunction
+
+%!shared part
+%! part = 0.514623842592593;
+%!assert(datenum(2001,5,19), 730990)
+%!assert(datenum([1417,6,12]), 517712)
+%!assert(datenum([2001,5,19;1417,6,12]), [730990;517712])
+%!assert(datenum(2001,5,19,12,21,3.5), 730990+part, eps)
+%!assert(datenum([1417,6,12,12,21,3.5]), 517712+part, eps)
+%!test
+%! t = [2001,5,19,12,21,3.5; 1417,6,12,12,21,3.5];
+%! n = [730990; 517712] + part;
+%! assert(datenum(t), n, 2*eps);