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));
|
