Bug with now function after DST changeover?

[AlanTheBeast]

I have a background program running in Terminal.  It does various "housekeeping" things for me.
One of the things is to initiate a Time Machine backup at specific times, currently set for noon (12:00*) and evening (18:00).

Today marks the first day of DST for the year and I noticed that:

1) The TM backup occurred at 13:00 rather than 12:00

2) The displayed time in the program is lagging by 1 hour.  That is to say, when it posts an event, the time is 1 hour behind the actual time that the system displays.

The function I use to check if it is time to start a function is the "now" function (returning a TDateTime).

So - it appears that this function is flawed in that it doesn't account for DST changing while a program is active.  The description of the now function is:

"Returns the current date and time."
https://www.freepascal.org/docs-html/rtl/sysutils/now.html


( MacOS 15.3.1, iMac M3,
  Free Pascal Compiler version 3.2.2 [2021/05/16] for aarch64)

Remedy for now is to re-start the housekeeping program. [I suppose I could program that in as well (if the program halts, then a "watch" program that is running will restart it).  So I could re-start the program on the DST change day.]
_____________________________________________________
*While above I use specific times, it's actually in the period after the time above, where the function is running about 1 per minute, so the backup could start at (say) 12:00:35.234 .

[cdbc]

Hi
I use something like the following to care about 'Now':

Code: Pascal  [Select][+][-]

1. (* anHour = 0.0834; { zulu + 0.0417 = local time ~ dk => summertime + 2 ~ 0.0834 } *)
2. function bcNow: TDateTime;
3. var lnow: TDateTime;
4. begin { wintertime is only 1 hour ahead of zulu in denmark }
5.   {$ifdef VER3_3_1} lnow:= NowUTC + anHour; {$else} lnow:= Now + anHour; {$endif}
6.   { summertime is 2 hours ahead of zulu in denmark ~ tz+dst }
7.   Result:= lnow + (DateIsInDST(lnow) * anHour); { func returns 0 or 1 }
8. end; { bcNow }

I have a function that returns 0 or 1 depending on the date being inside the DST window...
You're not alone in your struggle 
Regards Benny

[AlanTheBeast]

Hi
I use something like the following to care about 'Now':

Code: Pascal  [Select][+][-]

1.  
2. [code=pascal] {$ifdef VER3_3_1} lnow:= NowUTC + anHour; {$else} lnow:= Now + anHour; {$endif}
3.   { summertime is 2 hours ahead of zulu in denmark ~ tz+dst }
4.   Result:= lnow + (DateIsInDST(lnow) * anHour); { func returns 0 or 1 }

I have a function that returns 0 or 1 depending on the date being inside the DST window...
You're not alone in your struggle 
Regards Benny

Thank you.

Interesting - but I'm a little confused - if I run a fresh instance of my program after the DST change, then now returns the correct (DST) time.

So your above fix is for a program running at the time the change occurs?

Also (it would appear from a quick test) I would have to install later FPC to access NowUTC and DateIsInDST(t).

(I'm still on FPC 3.2.2).

[cdbc]

Hi
You're welcome.
It adapts every time you call it, using these:

Code: Pascal  [Select][+][-]

1. { used for daylight saving time }
2. function LastSundayInMarch(aYear: word): TDateTime;
3. var Dt: TIsoDate;
4. begin
5.   Dt:= TIsoDate.Create(24,03,aYear);
6.   try
7.     while Dt.DayNumber <> 7 do Dt.IncrementDay(1);
8.     Result:= Dt.AsDate;
9.   finally Dt.Free; end;
10. end;
11.  
12. { used for daylight saving time }
13. function LastSundayInOctober(aYear: word): TDateTime;
14. var I: IIsoDate;
15. begin
16.   I:= TIsoDate.Create(24,10,aYear);
17.   while I.DayNumber <> 7 do I.IncrementDay(1);
18.   Result:= I.AsDate;
19.   I:= nil;
20. end;
21.  
22. { DateIsInDST returns 1 for daylight saving time, 0 for normal time 15.08.2022 /bc }
23. function DateIsInDST(const aDateTime: TDateTime): integer;     { one could use }
24. begin                                                   { DateTimeInRange from }
25.   Result:= 0;                                                  { DateUtils.inc }
26.   if (aDateTime >= LastSundayInMarch(YearOf(aDateTime))) and
27.      (aDateTime < LastSundayInOctober(YearOf(aDateTime))) then Result:= 1;
28. end;

It works just fine when running /over/ the change, I couldn't be bothered to remember to restart an app at specific times, what are computers for?!? 
Regards Benny

eta: I run both 3.2.2 and trunk, so yes that's why the 'ifdef' is there...
     NowUTC is present in trunk.

[AlanTheBeast]

Thanks cdbc.  I'll review all that and implement it in some way.

[dbannon]

I use -

Code: Pascal  [Select][+][-]

1.     {$ifdef LINUX}
2.     ReReadLocalTime();    // in case we are near daylight saving time changeover
3.     {$endif}
4.     ThisMoment:=Now;

     

The ReReadLocalTime() is in the Unix unit, looks like my $ifdef should say "Unix" rather than "Linux"   

Davo

[AlanTheBeast]

I use -

Code: Pascal  [Select][+][-]

1.     {$ifdef LINUX}
2.     ReReadLocalTime();    // in case we are near daylight saving time changeover
3.     {$endif}
4.     ThisMoment:=Now;

     

The ReReadLocalTime() is in the Unix unit, looks like my $ifdef should say "Unix" rather than "Linux"   

That's interesting.  If I understand:
https://www.freepascal.org/docs-html/rtl/unixutil/epochtolocal.html

... I'll just add it to an existing function that runs once per hour, and it should take care of the issue in the future.

I don't bother with the portability directives, this is "hobby" code for my computer alone.

Thanks!