FIX: Daylight Savings Time Bug in C Run-Time Library

• Microsoft Visual C++, 32-bit Editions, versions 4.1, 4.2, 5.0, 6.0

SYMPTOMS

The C-Run-time Library date/time functions might fail to calculate the correct time during the first week of daylight savings time beginning April 1, 2001. The bug corrects itself after one week, on the following Sunday. This bug is not related to the year 2000 problem.

CAUSE

The bug is caused by a logic error in the C-Run-time library's cvtdate helper function.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article.This bug was corrected in Visual Studio 6.0 Service Pack 3. For more information about Visual Studio service packs, please see the following articles in the Microsoft Knowledge Base:

Q194022 INFO: Visual Studio 6.0 Service Packs, What, Where, Why

Q194295 HOWTO: Tell That Visual Studio 6.0 Service Packs Are Installed

MORE INFORMATION

Steps to Reproduce Behavior

// ----------------- start of sample program -------------------------

// Compiler switches needed : none.

#include <time.h>;
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>

int main ()
{
    struct tm ltm;
    time_t tstamp = 0;

    ltm.tm_year = 90;	/* First valid year .*/ 
    ltm.tm_mon = 3;		/* April. */ 
    ltm.tm_mday = 1;	/* 1st of the month. */ 
    ltm.tm_hour = 3;	/* At 3:00 am. */ 
    ltm.tm_min = 0;
    ltm.tm_sec = 0;
    ltm.tm_wday =0;
    ltm.tm_yday = 0;

    while (ltm.tm_year <= 2005)
    {
        ltm.tm_isdst = -1;

	// mktime indirectly calls cvtdate which has the logic bug.
	tstamp = mktime(&ltm);

	/* If it's Sunday April 1st, it should also be DST */ 
	if (!ltm.tm_wday && !ltm.tm_isdst)
	{
	    printf("April 01 is the first Sunday in April, %d  \ 
            -it should be DST but tm_isdst says it's \ 
            not!\n", 1900+ltm.tm_year);
	    while (!ltm.tm_isdst)
	    {
		ltm.tm_mday++;
		ltm.tm_isdst=-1;
		tstamp = mktime(&ltm);
	    }
	    printf("\tDST is reported as starting on  \ 
            %d/%d/%d\n", 1+ltm.tm_mon, ltm.tm_mday, \ 
	    1900+ltm.tm_year);
	
            ltm.tm_mday = 1;
	}
	
        ltm.tm_year++;
	
    }

    return 0;
}


// ----------------- end  of sample program --------------------------- 

REFERENCES

http://msdn.microsoft.com/visualc/headlines/2001.asp

Additional query words: y2k year2000

Keywords : kbservicepack kbCRT kbVC500 kbVC600bug kbVS600sp2 kbVS600SP1 kbVS600sp3fix
Version : winnt:4.1,4.2,5.0,6.0
Platform : winnt
Issue type : kbbug