Monday, March 27, 2006

Day 328

A trifle bizarre. After a bunch of investigation, I eventually narrowed down a synchronization problem to the following mini test program:

#include <time.h>
#include <stdio.h>
int main()
  time_t tv = time(NULL);
  struct tm *tms = localtime(&tv);
  printf("%02d:%02d:%02d DST:%d\n", 
         tms->tm_hour, tms->tm_min, 
         tms->tm_sec, tms->tm_isdst);
When I ran on Mac OS X:
Mon Mar 27 14:12:29 BST 2006
14:12:30 DST:1
But when I ran on my main Windows XP box:
c:\>time /t
13:13:03 DST:0
Now, I'm usually happy to believe the worst of Microsoft, but it stretches even my credibility to think that their C runtime doesn't get the 1989 ANSI standard right (particularly as a quick web search didn't turn up any complaints).

Investigating further, when I ran the same code on a different Windows XP machine, I get the right answer:

c:\>time /t
14:15:24 DST:0

So I checked MSDN and found some mutterings about "localtime corrects for the local time zone if the user first sets the global environment variable TZ...TZ is a Microsoft extension and not part of the ANSI standard definition of localtime. Note: The target environment should try to determine whether daylight saving time is in effect."

Running echo %TZ% from the command line showed up as GMT. So I tried changing the timezone to EST, but echo %TZ% was still reading GMT. Odd.

Eventually, I discovered that my login had TZ=GMT hard-coded into the environment (Control Panel, System, Advanced, Environment Variables). Not sure how it got that way, but finding it and making it not so has just taken a couple of hours.

[A:35029 B:3218 C:346 D:9187 Total:47780]

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