Calculating daylight saving time from only date
While it is easy to calculate whether a particular date is in DST for a particular location under the current rules, do note that DST is at the whim of politicians and could change at any point. I have a clock manufactured pre-2007 that automatically adjusts for daylight savings time, and now I have to change it four times a year: twice when the actual change occurs, and twice when it now-incorrectly changes itself at the old dates.
In this case, you might be able to ignore DST completely by the simple expedient of having the user enter the timezone along with the date and time. Or you could do like most consumer devices and let the user adjust the time to the local time zone twice a year.
But if you really need to handle DST and really want to do things right, use the zoneinfo database and make sure it can be updated somehow. If you can't do that for some reason, at least allow the user to override the rules. And if even that is too difficult, at least give the user the option to turn off automatic adjustments (unlike my stupid alarm clock).
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
***************/
Code for Central Europe (tested for every day in range 2014-3000 year)
public static bool IsDst(int day, int month, int dow)
{
if (month < 3 || month > 10) return false;
if (month > 3 && month < 10) return true;
int previousSunday = day - dow;
if (month == 3) return previousSunday >= 25;
if (month == 10) return previousSunday < 25;
return false; // this line never gonna happend
}
Test function
static void Main(string[] args)
{
TimeZoneInfo tzf2 = TimeZoneInfo.FindSystemTimeZoneById("Central Europe Standard Time");
var date = new DateTime(2014, 01, 1, 5, 0,0);
bool wasSummer = false;
while (date <= new DateTime(3000,1,1))
{
var dow = (int) date.DayOfWeek;
var isDst = IsDst(date.Day, date.Month, dow);
DateTime f2 = TimeZoneInfo.ConvertTime(date, tzf2);
var isSummer = f2.IsDaylightSavingTime();
if (isSummer != isDst)
{
Console.WriteLine("ERROR");
Console.WriteLine(date);
}
if (isSummer != wasSummer)
{
Console.WriteLine(date.AddDays(-1).ToShortDateString());
}
date = date.AddDays(1);
wasSummer = isSummer;
}
Console.ReadKey();
}
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).