days_diff.c

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#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <math.h>
#include <sys/time.h>
 
/* function to a date/time by a specified number of hours */
 
int adj_time(int adj, int y, int m, int d, int h,
            int *adj_y, int *adj_m, int *adj_d, int *adj_h)
{
 
int y1,m1,d1,h1;
 
  while(adj < -24)  
  {
    adj_time(-24,y,m,d,h,&y1,&m1,&d1,&h1);
    y=y1;m=m1;d=d1;h=h1;
    adj=adj+24;
  }
 
  while (adj > 24)
  {
    adj_time(24,y,m,d,h,&y1,&m1,&d1,&h1);
    y=y1;m=m1;d=d1;h=h1;
    adj=adj-24;
  }
 
  h=h+adj;
 
  if(h > 23)
  {
    h=h-24;
    d++;
    if((m == 1 || m == 3  || m == 5 || m == 7 || 
        m == 8 || m == 10 || m == 12 ) && d > 31)
    {
       d=1;
       m++;
       if(m > 12)
       {
         m=1;
         y++;
       }
    }
    if((m == 4 || m == 6 || m == 9 || m == 11) 
       && d > 30)
    {
      d=1;
      m++;
    }
 
    if(m == 2 && (((y%4) && d > 28) || (!(y%4) && d > 29)))
    {
      d=1;
      m++;
    }
  }  
  if(h < 0)
  {
    h=h+24;
    d--;
    if(d < 1)
    {
      m--;
      if(m == 0)
      {
        m=12;
        y--;
      }
      if(m == 1 || m == 3  || m == 5 || m == 7 || 
         m == 8 || m == 10 || m == 12 ) d=31;
      if(m == 4 || m == 6 || m == 9 || m == 11) d=30;
      if(m == 2 &&  y%4) d=28;
      if(m == 2 && !(y%4)) d=29;
    }
  }
 
  *adj_h=h;
  *adj_d=d;
  *adj_m=m;
  *adj_y=y;
  return(0);
}
 
/* function to identify zulu to local time offset */
int z2loc(int yy,int mm,int dd,int hh)
{
FILE *pf, *popen();
char day[80];
char line1[80];
int day_num;
int daylight=-7;
int standard=-8;
 
  if(mm>=4 && mm<=10)
    return(daylight);
  if(mm>=12 || mm<=2)
    return(standard);
 
  /* get day number */
  sprintf(line1,"date +\'%w\' -d \'%02d/%02d/%04d %02d:00\'",mm,dd,yy,hh);
  pf=popen(line1,"r");
  fgets(day,sizeof(day),pf);
  pclose(pf);
  sscanf(day,"%d",&day_num);
  if(!day_num) day_num=7;;
 
  /* November, 1st Sunday*/
  if(mm==11)
  {
    if(dd>=8)
      return(standard);
    if(day_num==7)
    {
      if(hh>=10)
        return(standard);
      else
        return(daylight);
    }
    else
    {
      if(day_num-dd<=0)
        return(standard);
      else
        return(daylight);
    }
  }
 
  /* March, second Sunday */
  if(mm==3)
  {
    if(dd<=7)
      return(standard);
    if(dd >=15)
      return(daylight);
    if(day_num==7)
    {
      if(hh>=10)
        return(daylight);
      else
        return(standard);
    }
    else
    {
      if(day_num-dd>=-7)
        return(standard);
      else
        return(daylight);
    }
  }
  return(0);
}