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Main Site Discussion => Posts => Topic started by: Dakusan on March 23, 2011, 10:54:31 pm

Title: Setting the current time zone through a numeric offset
Post by: Dakusan on March 23, 2011, 10:54:31 pm

Original post for Setting the time zone through a numeric offset can be found at https://www.castledragmire.com/Posts/Setting_the_time_zone_through_a_numeric_offset.
Originally posted on: 03/23/11

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));