How to implement daylight saving?

[aCkO]

Actually, this is a lot simpler than it might seem. As Dany already noted, you should keep the clock running as if it were GMT and add the necessary offset based on the DST settings. This way you can make your RTC app location independent, i.e. you get "world clock" functionality.

It all comes down to determining the dates of DST changes (for CET this is the last Sunday of March and the last Sunday of October). Obviously, the last Sunday will be between 25th and 31st of those months. So, for an arbitrary date and day of the week, calculating the date of previous Sunday will determine the DST flag. If we (conveniently) mark the days of the week like Sunday=0, Monday=1,..., Saturday=6, then our calculation of the previous Sunday becomes:

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PrevSunday = Day - DayOfTheWeek;

Our DST function then becomes:

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short IsDST(short day, short month, short dow) {
   if (month < 3 || month > 10) return 0;
   if (month > 3 && month < 10) return 1;
   
   if (month == 3) return day - dow >= 25;
   return day - dow < 25;
}

And that's it Of course, if the day of the week is Sunday and the current date is between 25th and 31st of March/October, you will also need to check if time is >= 2AM, but I will leave that to you.

Also, most RTC chips have DayOfTheWeek register, but you can also calculate it from the date (day, month and year):

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int dow(int y, int m, int d) {
   int t[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4};
   y -= m < 3;
   return (y + y/4 - y/100 + y/400 + t[m-1] + d) % 7;
}

or more compact version:

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int dow(int y, int m, int d) {
   return (d += m<3 ? y-- : y-2, 23*m/9 + d + 4 + y/4 - y/100 + y/400) % 7;
}

As in the previous example, Sunday=0, Monday=1, etc.

Regards

[Dany]

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short IsDST(short day, short month, short dow) {
   if (month < 3 || month > 10) return 0;
   if (month > 3 && month < 10) return 1;
   
   if (month == 3) return day - dow >= 25;
   return day - dow < 25;
}

Does this also work well if the last sunday of the month e.g. equals the 28th in stead of the 25th? I assume it does, but I do not see how it works...

[aCkO]

When you subtract (day - dow), you get the previous Sunday. If the result is between 25th and 31st (or simply >=25) then it is definitely the last Sunday of the month, so DST flag is on.
Let's say the last Sunday is 28th of March:

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Fri 26th  =>  dow = 5  =>  return (26 - 5 >= 25)  =>  False
Sat 27th  =>  dow = 6  =>  return (27 - 6 >= 25)  =>  False
Sun 28th  =>  dow = 0  =>  return (28 - 0 >= 25)  =>  True
Mon 29th  =>  dow = 1  =>  return (29 - 1 >= 25)  =>  True
Tue 30th  =>  dow = 2  =>  return (30 - 2 >= 25)  =>  True
Wed 31st  =>  dow = 3  =>  return (31 - 3 >= 25)  =>  True

Regards

[Dany]

When you subtract (day - dow), you get the previous Sunday. If the result is between 25th and 31st (or simply >=25) then it is definitely the last Sunday of the month, so DST flag is on.
Regards

Nice! Thanks
I did already suspect I did not see the working of your code due to my limited C knowledge...

[Dany]

Hi Acko, is it allowed for me to use the code you published in my own (mikroPascal) libraries (that are published under LibStock)? I will add a reference to your post(s) above. Thanks in advance!

[aCkO]

Hi Dany,

Of course, feel free to use this code anywhere you like I'm glad you found it useful to include it in your libraries

Regards

[Dany]

Hi Dany,

Of course, feel free to use this code anywhere you like I'm glad you found it useful to include it in your libraries

Regards

Thanks!

The mikroPascal code became:

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function RTC2Dst(Day, Date, Month: byte): boolean;
// see http://www.mikroe.com/forum/viewtopic.php?f=88&t=30565&start=2
var TmpDate, TmpMonth: byte; // decimal versions of the parameters
    I, TmpDay, Tmp: byte;
    SunDayFound: boolean;
begin

  // Transitions occur at
  // The last sunday of March (no DST --> DST) and
  // the last sunday of October (DST --> no DST)
  // Both transitions occur between dates 25..31 inclusive

  Result := false; // no DST (normal time)

  TmpMonth := BCD2DEC(Month); // make binary format

  if (TmpMonth < 3) or (TmpMonth > 10) then // no DST for sure
  begin
    // Result := false;
    exit;
  end;

  if (TmpMonth > 3) and (TmpMonth < 10) then // DST for sure
  begin
    Result := true;
    exit;
  end;


  // The month apparently is March or October, do the calculation

  TmpDate := BCD2DEC(Date);  // make binary format

  // ------------- the Acko solution -------------------------------------------
  // --- see http://www.mikroe.com/forum/viewtopic.php?f=88&t=30565&start=2 ----

  if Day = 7 then Day := 0; // Sunday needs to be zero for this calculation
  // here 0 = Sunday, 1 = Monday etc...

  Tmp := TmpDate - Day;

  if (TmpMonth = 3) then
  begin  // month = March
    if (Tmp >= 25) then // already summertime
    begin
      Result := true;
      exit;
    end;
  end
  else
  begin //  month = October
    if (Tmp < 25) then  // still summertime
    begin
      Result := true;
      exit;
    end;
  end;

end;

The above code could be written more dense I suppose, but it shows clearly what happens. See also the code (http://www.mikroe.com/forum/viewtopic.p ... 65&start=2) and explanation (http://www.mikroe.com/forum/viewtopic.p ... 65&start=4) provided by aCkO.