Calculating daylight saving time from only date
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
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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 <stdio.h> #include <string.h> #include <time.h> #include <sys/timeb.h> 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 <stdio.h> #include <string.h> #include <time.h> 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 ***************/讨论(0) -
This is actually deceptively simple. There are a few facts that will help us:
- In most of the US, DST starts on the second Sunday of March and ends on the first Sunday of November, at 2:AM both times.
- The second Sunday in March will always be between the 8th and the 14th inclusive.
- The first Sunday in November will always be between the 1st and 7th inclusive.
- The day of week numbering is quite convenient because the day - day of week will give you the previous Sunday.
These facts lead to the following code (C#, but trivially portable to your platform):
public bool IsDST(int day, int month, int dow) { //January, february, and december are out. if (month < 3 || month > 11) { return false; } //April to October are in if (month > 3 && month < 11) { return true; } int previousSunday = day - dow; //In march, we are DST if our previous sunday was on or after the 8th. if (month == 3) { return previousSunday >= 8; } //In november we must be before the first sunday to be dst. //That means the previous sunday must be before the 1st. return previousSunday <= 0; }It turns out you don't even need to know the year to do this, as long as you can trust your day of the week value.
I wrote a quick unit test and verified that this code agrees with
TimeZone.IsDayLightSavingsTime()for all dates from 1800 to 2200. I did not account for the 2 am rule, but you could easily do that check if the day of week is Sunday and the date is between 8 and 14 (in March) or 1 and 7 (in November).讨论(0) -
I found this all very helpful. Brazil has some special issues, southern hemisphere, and sometimes Carnival overlaps the fall change date.
In these cases, the legislature delays DST by one week. The US Naval observatory calculation can find easter, (at http://aa.usno.navy.mil/faq/docs/easter.php, retrieved 1/3/2017), and carnival is an exact number of days earlier, the week-end before ash wednesday (Mardi Gras means "fat tuesday").
So, in C:
static const uint8_t carnival[] = { 0x04, 0x24, 0x24, 0x21, // 2000... 2031 0x01, 0x09, 0x48, 0x09, // 2032... 2063 0x4a, 0x40, 0x4a, 0x52, // 2064... 2095 0x02, 0x90, 0x12, 0x94 // 2096... 2127 } /* Returns the current time offset. */ int dst(struct tm *tm_ptr) { int st = 0; int dst = 60; int mon = tm_ptr->tm_mon; int mday, previous_sunday; int gmt_offset = tm_ptr->gmt_offset; // If not Brasilia or Amazon time, no DST. if(gmt_offset != -240 && gmt_offset != -300) return st; if(NOV < mon || FEB > mon) // Summer? return dst; else if(NOV > mon && FEB < mon) // Winter? return st; mday = tm_ptr->tm_mday; previous_sunday = mday - tm_ptr->tm_wday; // Begin DST on first Sunday of November. if(NOV == mon) // If it's November... i.e. spring, so forward { if(previous_sunday < 1) // Before Sunday, week 1? { return st; } else { // After or during Sunday, week 1 return dst; } // End DST in February, accounting for Carnival. } else { // It has to be February, i.e. fall, so backward. int year, week_start; year = tm_ptr->tm_year; if(0 == (carnival[year/8] & (1 << (year%8)))) week_start = 15; // 3rd Sunday is not in Carnival. else week_start = 22; // Use 4th Sunday, 1 week after Carnival. if(previous_sunday < (week_start-1)) return dst; if(previous_sunday < week_start) { if(tm_ptr->tm_isdst == st) // if it already fell backward, stay. return st; return dst; } // On or after the correct Sunday? return st; } }讨论(0)
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