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Setting the time zone through a numeric offset
They never make it easy
Time: 03/24/11 1:53:57 am
Tags: Time Zone, Perl [3], Formats [6], MySQL [8], C [8], PHP [18], Examples [74]
Most relevent 4 of 89 posts with shared tags:

I had the need today to be able to set the current time zone for an application in multiple computer languages by the hourly offset from GMT/UTC, which turned out to be a lot harder than I expected. It seems most time zone related functions, at least in Linux, expect you to use full location strings to set the current time zone offset (i.e. America/Chicago).


After a lot of research and experimenting, I came up with the following results. All of these are confirmed working in Linux, and most or all of them should work in Windows too.

Language Format Note Format for GMT+5 Format for GMT-5
C Negate GMT-5 GMT5
Perl Negate GMT-5 GMT5
SQL Requires Sign +5:00 -5:00
PHP Negate, Requires Sign Etc/GMT-5 Etc/GMT+5

And here are examples of using this in each language. The “TimeZone” string variable should be a 1-2 digit integer with an optional preceding negative sign:
Language Example
C
#include <stdio.h> //snprintf
#include <stdlib.h> //setenv, atoi
#include <time.h> //tzset

...

char Buffer[10];
snprintf(Buffer, 10, "GMT%i", -atoi(TimeZone));
setenv("TZ", Buffer, 1);
tzset();
		
Perl
use POSIX qw/tzset/;
$ENV{TZ}='GMT'.(-$TimeZone);
tzset;
		
SQL [Query string created via Perl]
$Query='SET time_zone="'.($TimeZone>=0 ? '+' : '').$TimeZone.':00"';
		
PHP
date_default_timezone_set('Etc/GMT'.($TimeZone<=0 ? '+' : '').(-$TimeZone));
		

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