FIX: localtime() Does Not Always Switch to Standard Time

The information in this article applies to:

• The C Run-time (CRT) included with:

      - Microsoft Visual C++ for Windows, versions 1.5, 1.51, 1.52
      - Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2, 4.0
  

SYMPTOMS

The C run-time function localtime() incorrectly fills the tm_isdst member of the returning struct tm when it is executed in a time zone that does not switch from Daylight Savings to Standard time on the same date that the U.S. time zones make the switch.

CAUSE

The C Runtime Function localtime() is apparently not considering the time zone. Instead, it is assuming that the switch to Standard time always occurs on the last Sunday in October.

RESOLUTION

Choose one of the following two workarounds:

• Use the return value of the Win32 SDK function GetTimeZoneInformation, which returns a DWORD. A return value of TIME_ZONE_ID_STANDARD or TIME_ZONE_ID_DAYLIGHT will indicate whether the system time has been adjusted for daylight savings time. GetTimeZoneInformation correctly considers the current system time zone settings and adjusts on the proper date. (See the documentation for GetTimeZoneInformation, as there are other possible return values.)

  -or-

• Obtain the source for the C Run-time Library, and modify the localtime function to adjust according to the current system time zone setting. The C Run-time Library source is available in several locations. For instructions about how to obtain the C Runtime Library Source, please see the following article in the Microsoft Knowledge Base:

  For Visual C++ 2.x:

        ARTICLE-ID: Q123464
        TITLE     : How to Obtain Visual C++ 2.0 Run-Time Library Source Code
  
     NOTE: localtime is defined in the file Localtim.c. Windows NT and
     Windows 95 both account for the different dates by way of the
     HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\TimeZoneInformation
     registry entry, which is changed when the time zone changes in the
     control panel.
  
  

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 Microsoft Visual C++, 32-bit Edition, version 4.1.

MORE INFORMATION

Some Time Zones switch from Daylight Savings back to Standard Time about a month before most time zones (including the U.S. time zones). GMT + 0200 (Cairo) is an example. In the GMT + 0200 time zone, the switch date is the last Wednesday in September, whereas most other time zones switch on the last Sunday in October.

Calling the localtime() CRT to fill in a structure of type tm (defined in Time.h) gives an incorrect result of non-zero for the tm_isdst data member if you call it from one of the time zones that switch early on a date that falls between the two. A non-zero value in the tm_isdst member indicates that the system time has been adjusted for daylight savings time.

Sample Code

/* Compile options needed: None
*/

#include <iostream.h>
#include <time.h>

void main()

  tm *loc;
  time_t clock;

  time(&clock);
  tzset();
  loc = localtime(&clock);
  cout << "In timezone GMT+0200, 9-26-1995 is DST, ";
  cout << "9-27-1995 is not DST" << endl;
  cout << "The current date and time is: " << asctime(loc) << endl;
  if(loc->tm_isdst)
    cout << "It is daylight savings time" << endl;
  else
    cout << "It isn't daylight savings time" << endl;

Steps to Reproduce Problem

1. Change your system date to 9/26/95.

2. Change your time zone to GMT + 0200. Note that Egypt Daylight Time displays as your time zone. (Windows 95 and Windows NT automatically adjust the time.)

3. Run the sample code listed above, and note that it correctly says that it is daylight savings time.

4. Change the system date to 9/27/95. Note that Egypt Daylight Time changes to Egypt Standard. (Windows 95 and Windows NT automatically adjust the time.)

5. Run the sample code again, and note that it incorrectly says that it is daylight savings time.