GSM Time to Unix

gsmtime.c

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

#ifndef TM_YEAR_BASE
#define TM_YEAR_BASE 1900
#endif

#ifndef GSM_YEAR_BASE
#define GSM_YEAR_BASE 2000
#endif

#define GSM_MINUTES_PER_TZ_OFFSET 15

#ifndef SECSPERMINUTE
#define SECSPERMINUTE 60
#endif

static uint8_t decode_ascii_hex_char(char x)
{
   if (x >= 'a')
   {
      // Shift to uppercase (-0x20)
      // Remove gap from '9' to 'A' (-7)
      // Remove '0' offset (-0x30)
      return (x - (0x20 + 0x07 + 0x30));
   }
   if (x >= 'A')
   {
      // Remove gap from '9' to 'A' (-7)
      // Remove '0' offset (-0x30)
      return (x - (0x07 + 0x30));
   }
   // Remove '0' offset (-0x30)
   return (x - (0x30));
}

time_t scts_to_unix(struct tm *dest, const uint8_t *src)
{
   // We've been given a time as a 14-character (7-byte) ASCII/binary-coded-decimal string, in this format:
   // https://en.wikipedia.org/wiki/GSM_03.40#Time_Format
   // NOTE: This function doesn't require the time string be null-terminated, and will only look at the 14-bytes 

   // We want to convert it to a POSIX struct tm

   uint16_t gsm_year;
   uint8_t gsm_month;
   uint8_t gsm_day;
   uint8_t gsm_hour;
   uint8_t gsm_minute;
   uint8_t gsm_second;
   uint8_t temp_decode;
   int8_t gsm_timezone;
   struct tm tm_struct;
   time_t timezone_offset;

   if (dest == NULL)
   {
      return -1;
   }
   
   gsm_year          = decode_ascii_hex_char(src[0]) + (decode_ascii_hex_char(src[1]) * 10) + GSM_YEAR_BASE;
   gsm_month         = decode_ascii_hex_char(src[2]) + (decode_ascii_hex_char(src[3]) * 10);
   gsm_day           = decode_ascii_hex_char(src[4]) + (decode_ascii_hex_char(src[5]) * 10);
   gsm_hour          = decode_ascii_hex_char(src[6]) + (decode_ascii_hex_char(src[7]) * 10);
   gsm_minute        = decode_ascii_hex_char(src[8]) + (decode_ascii_hex_char(src[9]) * 10);
   gsm_second        = decode_ascii_hex_char(src[10]) + (decode_ascii_hex_char(src[11]) * 10);
   temp_decode = decode_ascii_hex_char(src[13]);
   if (temp_decode >= 8) {
       temp_decode -= 8;
       gsm_timezone = decode_ascii_hex_char(src[12]) + (temp_decode * 10);
       gsm_timezone = -gsm_timezone;
   }
   else {
       gsm_timezone = decode_ascii_hex_char(src[12]) + (temp_decode * 10);
   }
   timezone_offset = gsm_timezone * GSM_MINUTES_PER_TZ_OFFSET * SECSPERMINUTE;
   
   dest->tm_sec = gsm_second;   // seconds after the minute 0-59
   dest->tm_min = gsm_minute;   // minutes after the hour  0-59
   dest->tm_hour = gsm_hour;  // hours since midnight    0-23
   dest->tm_mday = gsm_day;  // day of the month        1-31
   dest->tm_mon = gsm_month - 1;   // months since January    0-11
   dest->tm_year = gsm_year - TM_YEAR_BASE;  // years since 1900
   dest->tm_wday = 0;
   dest->tm_yday = 0;
   dest->tm_isdst = -1;
   return timezone_offset;
}

void demo_gsmtime(void)
{
   char time_string_a[14] = "3130523210658A";
   struct tm tm_struct;
   time_t tz_offset;
   printf("GSM/SCSTS: %s\n",time_string_a);
   tz_offset = scts_to_unix(&tm_struct, time_string_a);
   printf("asctime: %s",asctime(&tm_struct));
   printf("tz_offset=%ld\n",tz_offset);
}