Microsoft KB Archive/214661

= FIX: Daylight Saving Time Bug in C Run-Time Library =

Article ID: 214661

Article Last Modified on 1/23/2007

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APPLIES TO


 * Microsoft Visual C++ 4.1 Subscription
 * Microsoft Visual C++ 4.2 Enterprise Edition
 * Microsoft Visual C++ 5.0 Enterprise Edition
 * Microsoft Visual C++ 6.0 Enterprise Edition
 * Microsoft Visual C++ 4.2 Professional Edition
 * Microsoft Visual C++ 5.0 Professional Edition
 * Microsoft Visual C++ 6.0 Professional Edition
 * Microsoft Visual C++ 6.0 Standard Edition

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This article was previously published under Q214661



SYMPTOMS
The C-Run-time Library date/time functions might fail to calculate the correct time during the first week of daylight saving 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 that this is a bug in the Microsoft products that are 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:

194022 INFO: Visual Studio 6.0 Service Packs, What, Where, Why

194295 HOWTO: Tell That Visual Studio 6.0 Service Packs Are Installed



Steps to Reproduce Behavior
The following sample program reproduces the problem: // - start of sample program -

// Compiler switches needed : none.


 * 1) include ;
 * 2) include 
 * 3) include 
 * 4) include 

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 ---

