Calculating daylight saving time from only date

Question

I am working with an Arduino and a real time clock chip. The chip compensates for leap years and such, so it will always have the correct date, but it does not handle daylight s

Answer 1

this code uses mktime to get a day of the week. It used a day of the week to compute daylight savings time. If you don't want to use mktime, you can use program second_sunday. Start with 3/14/2007, which is Wednesday. The day of the week will advance 1 day for every year and 2 days for every leap after 2004.

#include 
#include 
#include 
#include 


int isDst(int month, int dayOfMonth, int hour, int dayOfWeek);

int main(int argc, char *argv[])
{
   int isdst, dayOfWeek;
   char buf[80];
   struct tm tmData;

   if( argc == 1 )
   {
      printf("\nsyntax: %s mm/dd/yyyy_hh:mm:00", argv[0]);
      return -1;
   }

   // 0123456789A12
   // 03/12/2018_12
   strcpy(buf, argv[1]);
   tmData.tm_mon   = atoi(&buf[0]) - 1;  //month -1
   tmData.tm_mday  = atoi(&buf[3]); //day of month
   tmData.tm_year  = atoi(&buf[6]) - 1900; // year - 1900
   tmData.tm_hour  = atoi(&buf[11]); // hour
   tmData.tm_min   = 0; //minutes (not used)
   tmData.tm_sec   = 0; //seconds (not used)
   //tmData.tm_min   = atoi(&buf[14]);
   //tmData.tm_sec   = atoi(&buf[27]);

   //day light saving time variable.
   //NOT used in this calculation.
   //Tells mktime the input date is in day light saving time
   tmData.tm_isdst = 0; //

   mktime(&tmData);
   dayOfWeek = tmData.tm_wday;

   printf("%02d/%02d/%2d_%02d dayWk=%d ",
      tmData.tm_mon+1, tmData.tm_mday, tmData.tm_year, tmData.tm_hour, dayOfWeek);
   isdst = isDst(tmData.tm_mon+1, tmData.tm_mday, tmData.tm_hour, dayOfWeek);
   printf("isdst=%d\n", isdst);

   return 0;
}


int isDst(int month, int dayOfMonth, int hour, int dayOfWeek)
{
   int second_sunday, first_sunday;

   if( month  > 3 && month < 11  ) return 1; //4,5,6,7,8,9,10
   if( month  < 3 || month == 12 ) return 0; //1, 2 or 12
   if( month == 3 )
   {
      //The 2nd Sunday in March is 8,9,10,11,12,13,14
      if( dayOfMonth < 8  ) return 0;
      if( dayOfMonth > 14 ) return 1;

      //To get here dayOfMonth >= 8 && dayOfMonth <= 14
      second_sunday = dayOfMonth - dayOfWeek;
      if( second_sunday < 8 ) second_sunday += 7;
printf("2nd_Sunday=%2d ", second_sunday);
      if( dayOfMonth > second_sunday ) return 1;
      if( dayOfMonth < second_sunday ) return 0;

      //To get here dayOfMonth = second_sunday
      if( hour >= 2 ) return 1;
      else return 0;
   }

   if( month == 11 )
   {
      //The 1st Sunday in Nov is 1,2,3,4,5,6,7
      if( dayOfMonth > 7 ) return 0;

      //To get here dayOfMonth >= 1 && dayOfMonth <= 7
      first_sunday = dayOfMonth - dayOfWeek;
      if( first_sunday < 1 ) first_sunday += 7;
printf("1st_Sunday=%2d ", first_sunday);
      if( dayOfMonth > first_sunday ) return 0;
      if( dayOfMonth < first_sunday ) return 1;

      //To get here dayOfMonth = first_sunday
      if( hour >= 2 ) return 0;
      else return 1;
   }
   return -1;
}
/**************
   Compile via   cl.exe  isDst.c

   Begin and End dates for day light saving time
   03/11/2007_01:00:00    11/04/2007_01:00:00
   03/09/2008_01:00:00    11/02/2008_01:00:00
   03/08/2009_01:00:00    11/01/2009_01:00:00
   03/14/2010_01:00:00    11/07/2010_01:00:00
   03/13/2011_01:00:00    11/06/2011_01:00:00
   03/11/2012_01:00:00    11/04/2012_01:00:00
   03/10/2013_01:00:00    11/03/2013_01:00:00
   03/09/2014_01:00:00    11/02/2014_01:00:00
   03/08/2015_01:00:00    11/01/2015_01:00:00
   03/13/2016_01:00:00    11/06/2016_01:00:00
   03/12/2017_01:00:00    11/05/2017_01:00:00
   03/11/2018_01:00:00    11/04/2018_01:00:00
   03/10/2019_01:00:00    11/03/2019_01:00:00
   03/08/2020_01:00:00    11/01/2020_01:00:00
   03/14/2021_01:00:00    11/07/2021_01:00:00
   03/13/2022_01:00:00    11/06/2022_01:00:00
   03/12/2023_01:00:00    11/05/2023_01:00:00
   03/10/2024_01:00:00    11/03/2024_01:00:00
   03/09/2025_01:00:00    11/02/2025_01:00:00
   03/08/2026_01:00:00    11/01/2026_01:00:00
   03/14/2027_01:00:00    11/07/2027_01:00:00
   03/12/2028_01:00:00    11/05/2028_01:00:00
   03/11/2029_01:00:00    11/04/2029_01:00:00
   03/10/2030_01:00:00    11/03/2030_01:00:00
   03/09/2031_01:00:00    11/02/2031_01:00:00
   03/14/2032_01:00:00    11/07/2032_01:00:00

   isDst.exe 03/11/2007_02:00:00  >> dst.txt
   isDst.exe 03/09/2008_02:00:00  >> dst.txt
   isDst.exe 03/08/2009_02:00:00  >> dst.txt
   isDst.exe 03/14/2010_02:00:00  >> dst.txt
   isDst.exe 03/13/2011_02:00:00  >> dst.txt
   isDst.exe 03/11/2012_02:00:00  >> dst.txt
   isDst.exe 03/10/2013_02:00:00  >> dst.txt
   isDst.exe 03/09/2014_02:00:00  >> dst.txt
   isDst.exe 03/08/2015_02:00:00  >> dst.txt
   isDst.exe 03/13/2016_02:00:00  >> dst.txt
   isDst.exe 03/12/2017_02:00:00  >> dst.txt
   isDst.exe 03/11/2018_02:00:00  >> dst.txt
   isDst.exe 03/10/2019_02:00:00  >> dst.txt
   isDst.exe 03/08/2020_02:00:00  >> dst.txt
   isDst.exe 03/14/2021_02:00:00  >> dst.txt
   isDst.exe 03/13/2022_02:00:00  >> dst.txt
   isDst.exe 03/12/2023_02:00:00  >> dst.txt
   isDst.exe 03/10/2024_02:00:00  >> dst.txt
   isDst.exe 03/09/2025_02:00:00  >> dst.txt
   isDst.exe 03/08/2026_02:00:00  >> dst.txt
   isDst.exe 03/14/2027_02:00:00  >> dst.txt
   isDst.exe 03/12/2028_02:00:00  >> dst.txt
   isDst.exe 03/11/2029_02:00:00  >> dst.txt
   isDst.exe 03/10/2030_02:00:00  >> dst.txt
   isDst.exe 03/09/2031_02:00:00  >> dst.txt
   isDst.exe 03/14/2032_02:00:00  >> dst.txt
   isDst.exe 11/04/2007_02:00:00  >> dst.txt
   isDst.exe 11/02/2008_02:00:00  >> dst.txt
   isDst.exe 11/01/2009_02:00:00  >> dst.txt
   isDst.exe 11/07/2010_02:00:00  >> dst.txt
   isDst.exe 11/06/2011_02:00:00  >> dst.txt
   isDst.exe 11/04/2012_02:00:00  >> dst.txt
   isDst.exe 11/03/2013_02:00:00  >> dst.txt
   isDst.exe 11/02/2014_02:00:00  >> dst.txt
   isDst.exe 11/01/2015_02:00:00  >> dst.txt
   isDst.exe 11/06/2016_02:00:00  >> dst.txt
   isDst.exe 11/05/2017_02:00:00  >> dst.txt
   isDst.exe 11/04/2018_02:00:00  >> dst.txt
   isDst.exe 11/03/2019_02:00:00  >> dst.txt
   isDst.exe 11/01/2020_02:00:00  >> dst.txt
   isDst.exe 11/07/2021_02:00:00  >> dst.txt
   isDst.exe 11/06/2022_02:00:00  >> dst.txt
   isDst.exe 11/05/2023_02:00:00  >> dst.txt
   isDst.exe 11/03/2024_02:00:00  >> dst.txt
   isDst.exe 11/02/2025_02:00:00  >> dst.txt
   isDst.exe 11/01/2026_02:00:00  >> dst.txt
   isDst.exe 11/07/2027_02:00:00  >> dst.txt
   isDst.exe 11/05/2028_02:00:00  >> dst.txt
   isDst.exe 11/04/2029_02:00:00  >> dst.txt
   isDst.exe 11/03/2030_02:00:00  >> dst.txt
   isDst.exe 11/02/2031_02:00:00  >> dst.txt
   isDst.exe 11/07/2032_02:00:00  >> dst.txt
   https://stackoverflow.com/questions/5590429/calculating-daylight-saving-time-from-only-date
***************/



/*****
The previous programs used mktime to compute day_of_week.
It used day_of_week to compute 2nd_sunday in march and
1st_sunday in Nov.
If you don't want to use mktime, you can use this program to
compute 2nd_sunday.  The same technique will compute 1st_sunday.

On 03/14/2007, the day of the week is Wed, or 3.
Every year after 2007, the day of the week advances 1 day.
on leap years, the day of the week advances 2 days.
Must include the no. of leap years sinc 2004.
******/

#include 
#include 
#include 

int secondSunday(year);

int main(int argc, char *argv[])
{
   int year, second_sunday;

   if( argc == 1 )
   {
      printf("\nsyntax: %s year, with year >= 2007.\n", argv[0]);
      return -1;
   }

   year = atoi(argv[1]);
   if( year < 2007 )
   {
      printf("\nsyntax: %s year, with year >= 2007.\n", argv[0]);
      return -1;
   }

   second_sunday = secondSunday(year);
   printf("second_sunday=%d\n", second_sunday);

   return 0;
}


int secondSunday(year)
{
   //On 03/14/2007, the day of the week is Wed, or 3.
   int no_years, no_leaps, day_of_week, second_sunday;

   no_years = year - 2007;
   no_leaps = (year - 2004)/4;
   day_of_week = 3 + (no_years + no_leaps) % 7;
   second_sunday = 14 - day_of_week;
   if( second_sunday < 8 ) second_sunday += 7;

   //printf("no_years=%d,no_leaps=%d,day_of_week=%d, second_sunday=%d\n",
   //no_years, no_leaps, day_of_week, second_sunday);

   return second_sunday;
}
/**************
   Compile via   cl.exe  second_sunday.c

   second_sunday.exe 2007
   second_sunday.exe 2008
   second_sunday.exe 2009
   second_sunday.exe 2010
   second_sunday.exe 2011
   second_sunday.exe 2012
   second_sunday.exe 2013
   second_sunday.exe 2014
   second_sunday.exe 2015
   second_sunday.exe 2016
   second_sunday.exe 2017
   second_sunday.exe 2018
   second_sunday.exe 2019
   second_sunday.exe 2020
   second_sunday.exe 2021
   second_sunday.exe 2022
   second_sunday.exe 2023
   second_sunday.exe 2024
   second_sunday.exe 2025
   second_sunday.exe 2026
   second_sunday.exe 2027
   second_sunday.exe 2028
   second_sunday.exe 2029
   second_sunday.exe 2030
   second_sunday.exe 2031
   second_sunday.exe 2032
***************/