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

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 saving 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 saving time" << endl;
  else
    cout << "It isn't daylight saving 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 saving 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 saving time.