#include <stdio.h>
#include <time.h>

/*
 * http://groups.google.de/group/comp.unix.shell/msg/72bbc641422fdf73?dmode=source&hl=de
 */

/************************************************************************/
long        klok,                       /* date and time long integer   */
            calc_time();                /* time calculating procedure   */
char        format[80],                 /* formatting string            */
            *argv0,                     /* name of this command         */
            *getenv();                  /* environment procedure        */
/************************************************************************/
main (argc, argv)
int  argc;
char *argv[];
{
   char *cp;
   int  index;

   *format = 0;
   klok    = time(0L);
   argv0   = argv[0];

   /********** retreive offset and formating from command-line *********/
   for (index = 1; index < argc ; index++)
   {
      if (*argv[index] == '+')                  /* if parameter starts  */
      {                                         /* with a '+'           */
         cp = argv[index];              
         while (*cp != 0 && *cp != '%')         /* search for a '%'     */
           cp++;

         if  (*cp == '%')                       /* if a '%' found assume*/
            strcpy(format,argv[index]);         /* this is a format str.*/
         else                                   /* else                 */
            klok = calc_time(argv[index],klok); /* calculate new time   */
      }
      else
         if (*argv[index] == '-')
            klok = calc_time(argv[index],klok); /* calculate new time   */
         else 
            if (sscanf(argv[index],"%ld",&klok) < 0)
            {
               fprintf(stderr,"%s: Illegal argument\n",argv[index]);
               usage();
            }
   }

   printformatted(klok,format);
}
/************************************************************************/
/* Procedure OFFSET - Calculate offset                                  */
/************************************************************************/
long    calc_time(str,ck)
char    *str;
long    ck;
{
   int  sign = 1,                       /* sign of offset (pos./neg.)   */
        offset;                         /* nr. of days,hours,weeks...   */
   char type;                           /* type of offset               */

   if (*(str++) == '-')                 /* determine sign               */
     sign = -1;

   if (sscanf(str,"%u%c",&offset,&type) <= 0)
   {
      fprintf(stderr,"%s: Syntax error in offset\n",argv0);
      usage();
   }

   switch (type)
   {
   case 'd' : return(ck + sign *  86400 * offset);      /* days         */
   case 'h' : return(ck + sign *  3600 * offset);       /* hours        */
   case 'm' : return(ck + sign *  60 * offset);         /* minutes      */
   case 's' : return(ck + sign *  offset);              /* seconds      */

   default :
      fprintf(stderr,"%s: Unknown offset type \'%c\'\n",argv0,type);
      usage();
   }
}
/************************************************************************/
printformatted (cl,fo)
long  cl;
char  *fo;
{
   struct tm *timestruct;
   char *pntr, *timestring;

   timestruct = localtime(&cl);
   timestring = ctime(&cl);
   if (*fo)
   {
      for (pntr = fo + 1; *pntr ; pntr++)
      {
         if (*pntr == '%')
         {
            switch (*(++pntr))
            {
            case 'n' : printf("\n");                                    break;
            case 't' : printf("\t");                                    break;
            case 'm' : printf("%.2u",timestruct->tm_mon + 1);           break;
            case 'd' : printf("%.2u",timestruct->tm_mday);              break;
            case 'y' : printf("%.2u",timestruct->tm_year % 100);        break;
            case 'D' : printf("%.2u/%.2u/%.2u",timestruct->tm_mon + 1,
                          timestruct->tm_mday, timestruct->tm_year);
                       break;
            case 'H' : printf("%.2u",timestruct->tm_hour);              break;
            case 'M' : printf("%.2u",timestruct->tm_min);               break;
            case 'S' : printf("%.2u",timestruct->tm_sec);               break;
            case 'T' : printf("%.2u:%.2u:%.2u",timestruct->tm_hour,
                          timestruct->tm_min, timestruct->tm_sec);
                       break;
            case 'j' : printf("%.3u",timestruct->tm_yday);              break;
            case 'w' : printf("%.1u",timestruct->tm_wday);              break;
            case 'a' : printf("%3.-3s",timestring);                     break;
            case 'h' : printf("%3.-3s",timestring + 4);                 break;
            case 'r' : if (timestruct->tm_hour > 12)
                       {
                          printf("%.2u:",timestruct->tm_hour - 12);
                          printf("%.2u:",timestruct->tm_min);
                          printf("%.2u PM",timestruct->tm_sec);
                       }
                       else
                       {
                          printf("%.2u:",timestruct->tm_hour);
                          printf("%.2u:",timestruct->tm_min);
                          printf("%.2u AM",timestruct->tm_sec);
                       }
                       break;
            default:   usage();
            }
         }
         else
            printf("%c",*pntr);
      }
      printf("\n");
   }
   else
   {
      printf("%19.-19s ",timestring);
      printf("%3.-3s ",getenv("TZ"));
      printf("%4.-4s\n",&timestring[20]);
   }
}
/************************************************************************/
static usage ()
{
#define E(x)  fprintf(stderr,"%s\n",x)

   E(" Usage: xdate [CALCULATION] [+FORMAT]");
   E("CALCULATION can be used for date calculation. It can be:");
   E("+|- d|h|m|s   for +/- Days, Hours, Minutes, Seconds");
   E("FORMAT determines the print-format of the result. They are");
   E("%n - Newline");
   E("%t - Tab");
   E("%m - Month (01 -12)");
   E("%d - Day (01 - 31)");
   E("%y - YY (00 -99)");
   E("%D - MM/DD/YY");
   E("%H - Hour (00 - 24)");
   E("%M - Minute (00 - 60)");
   E("%S - Seconds (00 - 60)");
   E("%T - HH:MM:SS");
   E("%j - Daynumber in year (000 - 365)");
   E("%w - Daynumber in week (0 - 6, 0 = Sunday)");
   E("%a - Day name (Sun, Mon...)");
   E("%h - Month name (Jan, Feb..)");
   E("%r - HH:MM:SS [AM|PM]");
   exit(1);
}