NASA Ocean Color

ocssw V2022

 /*
  *----------------------------------------------------------------------
  *  @(#) julian.c   1.0 20 Mar 93   <shc>
  *  Copyright (c) 1993, CSIRO Division of Oceanography.
  *  Copyright (c) 1998, Datron Transco Inc..
  *----------------------------------------------------------------------
  *
  * julian --
  *
  *  A collection of routines for converting between normal time/date
  *  formats and Julian dates.
  *
  * History:
  *  25 Oct 93  <shc>
  *     Initial version
  *
  *  05 Jun 98  <shc>
  *     Divided into julday/caldat and other utility routines.
  *
  *----------------------------------------------------------------------
  */
  
 #include <math.h>
  
 #include "orbit.h" /* Prototype checking */
  
  
 /*
  *  This routine returns the Julian Day Number which begins at noon of
  *  the calendar date specified by the arguments.  Positive year signifies
  *  A.D., negative B.C.  Remember that the year after 1 B.C. was 1 A.D.
  */
  
  
 #define IGREG1 (15L + 31L * (10L + 12L * 1582L))
  
 int32_t julday(year, month, day)
 int year, month, day;
 {
     int32_t y, m, adj, date;
  
     if (year == 0)
         return (-1);
  
     if (year < 0)
         year += 1;
  
     if (month > 2) {
         y = year;
         m = month + 1;
     } else {
         y = year - 1;
         m = month + 13;
     }
  
     date = (int32_t) floor(365.25 * y) + (int32_t) (30.6001 * m) +
             (int32_t) day + 1720995L;
  
     if ((int32_t) day + 31 * ((int32_t) month + 12 * (int32_t) year) > IGREG1) {
         adj = (int32_t) (0.01 * y);
         date = date + 2 - adj + (int32_t) (0.25 * adj);
     }
  
     return (date);
 }
  
  
 /*
  *  Inverse of the function "julday" above.
  */
  
 #define IGREG2 2299161L
  
 void
 caldat(julian, year, month, day)
 int32_t julian;
 int *year, *month, *day;
 {
     int32_t ja, jb, jc, jd, je, jalpha;
  
     if (julian < IGREG2) {
         ja = julian;
     } else {
         jalpha = (int32_t) (((julian - 1867216L) - 0.25) / 36524.25);
         ja = julian + 1 + jalpha - (int32_t) (0.25 * jalpha);
     }
  
     jb = ja + 1524;
     jc = (int32_t) (6680.0 + ((jb - 2439870L) - 122.1) / 365.25);
     jd = 365 * jc + (int32_t) (0.25 * jc);
     je = (int32_t) ((jb - jd) / 30.6001);
  
     *day = jb - jd - (int32_t) (30.6001 * je);
     *month = je - 1;
     if (*month > 12)
         *month = *month - 12;
     *year = jc - 4715;
  
     if (*month > 2)
         *year = *year - 1;
     if (*year <= 0)
         *year = *year - 1;
  
     return;
 }