Re: NSCalendar bug (mktime related)

[Stefan Bidigaray]

Hi Fred/Andreas,

I wrote all that code, so I'm fairly familiar with it. The code is entirely dependent on ICU's implementation of ucal, so the solution is to simply set the Time Zone value in the ucal. The function to do this is: ucal_setTimeZone() (https://unicode-org.github.io/icu-docs/apidoc/released/icu4c/ucal_8h.html#ae5612988cb9dc282ccda82fda38602b2).

I no longer have a development machine with GNUstep installed, so I cannot make the changes and test it. There are two methods where this needs to be fixed:

-dateByAddingComponents:toDate: and -dateFromComponents:.

The part I'm not totally sure of is if the fix should be done by calling -setTimeZone: or ucal_setTimeZone(). Based on the method descriptions, I believe a call to ucal_setTimeZone should be added to lines NSCalendar.m:546 and NSCalendar.m:611 (first a UChar timezone name must be retrieved from the NSTimeZone object) since -setTimeZone: will permanently modify the NSCalendar object.

Regards

Stefan

On Sat, Apr 25, 2020 at 9:58 AM Fred Kiefer <fredkiefer@gmx.de> wrote:

Hi Andreas,

I checked the code of NSCalendar, which you could have done yourself, this is free software, and we are not using mtkime there. The problem is a lot worse. We just ignore the handed in time zone of the NSDateComponents. This could be repaired but I am no expert on NSCalendar so if anybody else wants to look into this feel free. It would be great to have a few more tests for NSCalendar. Anybody willing to provide a few test cases?

Fred

> Am 23.04.2020 um 10:51 schrieb Andreas Fink <list@fink.org>:
>
> Hello all,
>
> I have run into a very weird thing in conversion from NSStrings to NSDate. The result is we are always off by 1h under LInux.
> Under MacOS X I have the same problem but only with mktime() not with NSCalendar.
> I am suspecting Gnustep implementation probably uses mktime() in the back and thus inherits this issue also for NSCalendar.
>
> What I try to do is to convert aNSString with a timestamp which is always in UTC into a date.
> So the timestamp I supply has timezone GMT+0 and no daylight savings time.
> If the current system currently experiences daylight savings time, the result by mktime is off by 1h even if I specify timezone to be UTC in struct tm.
>
> Here is a test programm for this:
>
>
> #define _BSD_SOURCE
> #include <time.h>
> #include <string.h>
> #include <stdio.h>
> time_t test(void)
> {
>    struct tm tm;
>       memset(&tm,0, sizeof(tm));
>
>     int year = 1970;
>     int month = 1;
>     int day = 1;
>     int hour = 0;
>     int minute = 0;
>     int second = 0;
>       
>     tm.tm_year = year -1900;
>       tm.tm_mon = month -1,
>     tm.tm_mday = day;
>     tm.tm_hour = hour;
>     tm.tm_min = minute;
>     tm.tm_sec = second;
>     tm.tm_zone = "UTC";
>     tm.tm_isdst = -1;
>     tm.tm_gmtoff = 0;
>       
>     time_t t = mktime(&tm);
>       return t;
> }
>
>
> int main(int argc, const char * argv[])
> {
>       time_t t;
>
>       setenv("TZ","UTC",1);
>       t = test();
>       printf("TZ=UTC:  t=%d\n",t);
>
>       setenv("TZ","CET",1);
>       t = test();
>       printf("TZ=CET:  t=%d\n",t);
>
>       setenv("TZ","CEST",1);
>       t = test();
>       printf("TZ=CEST:  t=%d\n",t);
>
>       setenv("TZ","Europe/Zurich",1);
>       t = test();
>       printf("TZ=Europe/Zurich:  t=%d\n",t);
>
> }
>
>
> So the timestamp I supply has timezone GMT+0 and no daylight savings time.
> So the time_t value returned should always be 0 but its off by -1h if the envirnment has the timezone set to Europe/Zurich or CET. Interestingly CEST is correct (which is the current timezone in Zurich CET + Daylight savings time)
>
> TZ=UTC:  t=0
> TZ=CET:  t=-3600
> TZ=CEST:  t=0
> TZ=Europe/Zurich:  t=-3600
>
> The output of this is indicating that the daylight savings yes/no  from the supplied timezone is ignore as well as the timezone. It always bases the date on the current environmental variable even though the current timezone might not be the one in effect at that date.
> So it assumes that on 1.11970 we had daylight savings time (because TZ says we have now) despite being in January and despite it wasn't in use in that year even.
>
> Now lets say this is a bug of mktime or maybe a wanted feature. Be is at it is.
>
> The problem is NSCalendar fails for the same issue.
>
>
> This piece of code works under MacOS X but fails under Linux.
> Under NSCalendar I specify the timezone explicitly and TZ environment variable should not be relevant. But if NSCalendar implementation uses mktime, it inherits above strange behaviour.
>
> NSDate *dateFromStringNSCalendar(NSString *str, const char *ctimezone_str) /* expects YYYY-MM-DD hh.mm.ss.SSSSSS TZ  timestamps */
> {
>     int year;
>     int month;
>     int day;
>     int hour;
>     int minute;
>     int seconds;
>     double subsecond = 0;
>     const char *cdate_str;
>     const char *ctime_str;
>     
>     NSArray *components = [str componentsSeparatedByString:@" "];
>     if(components.count >0)
>     {
>         NSString *s = components[0];
>         cdate_str = s.UTF8String;
>     }
>     if(components.count > 1)
>     {
>         NSString *s = components[1];
>         ctime_str = s.UTF8String;
>     }
>     if(components.count > 2)
>     {
>         NSMutableArray *arr =  [components mutableCopy];
>         [arr removeObjectsInRange:NSMakeRange(0,2)];
>         NSString *s = [arr componentsJoinedByString:@" "];
>         ctimezone_str = s.UTF8String;
>     }
>
>     /* parsing date */
>     sscanf(cdate_str,"%04d-%02d-%02d",
>            &year,
>            &month,
>            &day);
>     if(strlen(ctime_str) ==8 ) /* HH:mm:ss.SSSSSS */
>     {
>         sscanf(ctime_str,"%02d:%02d:%02d",
>                &hour,
>                &minute,
>                &seconds);
>     }
>     else if(strlen(ctime_str) >=9  ) /* HH:mm:ss.SSSSSS */
>     {
>         sscanf(ctime_str,"%02d:%02d:%lf",
>                &hour,
>                &minute,
>                &subsecond);
>         seconds = (int)subsecond;
>         subsecond = subsecond - (double)seconds;
>     }
>     else
>     {
>         return NULL;
>     }
>     NSDateComponents *dateComponents = [[NSDateComponents alloc] init];
>     dateComponents.day = day;
>     dateComponents.month = month;
>     dateComponents.year = year;
>     dateComponents.hour = hour;
>     dateComponents.minute = minute;
>     dateComponents.second = seconds;
> #ifdef __APPLE__
>     dateComponents.nanosecond = subsecond * 1000000000;
> #endif
>     if(ctimezone_str!=NULL)
>     {
>         NSTimeZone *tz      = [NSTimeZone timeZoneWithName:@(ctimezone_str)];
>         dateComponents.timeZone = tz;
>     }
>     NSCalendar *gregorianCalendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
>     NSDate *date = [gregorianCalendar dateFromComponents:dateComponents];
>     return date;
> }
>
>
>
>
>