FIX: time() Function Overcompensates for Daylight Savings Time

• The C Run-Time (CRT), included with:
  • Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.0a, 4.1, 4.2, 4.2b

SYMPTOMS

The C run-time library time() function may, in some geographic locations and at certain times, return a time that is off by one hour to the actual time. This problem is specific to areas that do not use Daylight Savings Time (Japan, for example, or in parts of Indiana in the United States), and the problem occurs only at times at the beginning or end of Daylight Savings Time.

CAUSE

The time() function calls GetLocalTime, which returns the exact time, compensated for time zones and Daylight Savings Time. However, before the function ends it also calls the isindst() function. This function uses a standard formula for determining if the current date and time are part of Daylight Savings Time. If isindst() returns TRUE, the time is altered by 3600 seconds (one hour).

RESOLUTION

This problem has been corrected in Microsoft Visual C++ versions 5.0 and later. If you cannot upgrade to Visual C++ 5.0, there are two workarounds to this problem:

• Workaround 1. Call GetLocalTime() instead of time(). This method is documented in the following article in the Microsoft Knowledge Base:

  Q99456 Win32 Equivalents to C Run-Time Functions




• Workaround 2. If possible, start the Date/Time application and clear the check box to "Automatically adjust clock for daylight saving changes" (please note that this workaround may not be available in all areas). When time() is called, GetLocalTime will return the time adjusted for the local time zone, and isindst will cause time() to adjust the time for Daylight Savings Time.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug has been corrected in Microsoft Visual C++, 32-bit edition, versions 5.0 and later.

MORE INFORMATION

To reproduce this problem:

1. Compile the sample code below using Visual C++ 4.x.



2. Open the Date/Time application and make the following changes:
   
   
   • Set the time zone to GMT + 9:00 (Tokyo, Osaka, Sapporo, Seoul, Yakutsk).
   
   
   
   • Set the time to October 25, 11:55 PM.
   
   
   



3. Start running the application on Windows NT before the clock changes to midnight, and stop running the application several seconds after midnight.



4. Open the output file generated by the program. Here is a brief section of the output you should see:

   
         877849195
         877849196
         877849197
         877849198
         877849199
         877852800  << this time is 3600 seconds, or 1 hour off
         877849201
                .
                .
                . 

/* Compiler options needed: none */ 

#include <time.h>
#include <stdio.h>
#include <fstream.h>
#include <windows.h>

int main(void)
{
  time_t t1;
  ofstream ostr("timebug.txt");

  for (;;)
  {
    time(&t1);
    ostr << t1 << endl;
    Sleep(1000);
  }
  ostr.close();
  return 0;
} 

Additional query words: time mktime asctime _ftime gmtime tzset

Keywords : kbcode kbCRT kbVC400fix
Version : WINNT:4.0,4.0a,4.1,4.2,4.2b
Platform : winnt
Issue type : kbbug