TimeZone setting

In our DVR, we have an option for user to decide whether Daylight Saving Time (DST) to be enabled in his machine.  When Time Zone is selected in DST field, the time in Date/Time fields will be automatically adjusted accordingly. However, once the user changed the Date/Time value, the Time Zone changed later will not effect the time in Date/Time field.

Ref:
[1] http://www.gnu.org/software/libc/manual/html_node/Time-Zone-Functions.html


[0] Find the time zone table in your system
# cat /usr/share/zoneinfo/zone.tab


# <pre>
# @(#)zone.tab 8.38
# This file is in the public domain, so clarified as of
# 2009-05-17 by Arthur David Olson.
#
# TZ zone descriptions
#
# From Paul Eggert (1996-08-05):
#
# This file contains a table with the following columns:
# 1.  ISO 3166 2-character country code.  See the file `iso3166.tab'.
# 2.  Latitude and longitude of the zone's principal location
#     in ISO 6709 sign-degrees-minutes-seconds format,
#     either +-DDMM+-DDDMM or +-DDMMSS+-DDDMMSS,
#     first latitude (+ is north), then longitude (+ is east).
# 3.  Zone name used in value of TZ environment variable.
# 4.  Comments; present if and only if the country has multiple rows.
#
# Columns are separated by a single tab.
# The table is sorted first by country, then an order within the country that
# (1) makes some geographical sense, and
# (2) puts the most populous zones first, where that does not contradict (1).
#
# Lines beginning with `#' are comments.
#
#country-
#code coordinates TZ comments
AD +4230+00131 Europe/Andorra
AE +2518+05518 Asia/Dubai
AF +3431+06912 Asia/Kabul
AG +1703-06148 America/Antigua
AI +1812-06304 America/Anguilla
AL +4120+01950 Europe/Tirane
AM +4011+04430 Asia/Yerevan
AN +1211-06900 America/Curacao
AO -0848+01314 Africa/Luanda
AQ -7750+16636 Antarctica/McMurdo McMurdo Station, Ross Island
AQ -9000+00000 Antarctica/South_Pole Amundsen-Scott Station, South Pole
AQ -6734-06808 Antarctica/Rothera Rothera Station, Adelaide Island
AQ -6448-06406 Antarctica/Palmer Palmer Station, Anvers Island
AQ -6736+06253 Antarctica/Mawson Mawson Station, Holme Bay
AQ -6835+07758 Antarctica/Davis Davis Station, Vestfold Hills
          :
          :


What we are interested are TZ and comments :) We may show the comments in our UI timezone list for user to select his own timezone.


[1] To set your timezone

Method#1:
# export TZ=Asia/Taipei

To get timezone setting
# echo $TZ

Method#2:

If TZ is empty or not set, /etc/localtime is used.

So, we can use symbolic link as below to set timezone.

# ln -s /usr/shar/zoneinfo/Asia/Taipei /etc/localtime

or 

# export TZ=/etc/localtime

   /etc/localtime is a symbolic link to your timezone file.

[2] For C user:
  ...
  some UI work to get the timezone to be set....
  ...
  setenv("TZ", "Asia/Taipei", 1); /// "Asia/Taipei" is the timezone you may want to set