[tz] CST timezone that does not account for DST

[Paul Eggert]

Nilesh Parmar wrote:
> Using the POSIX string, we can create a timezone called CST that is GMT+6 as follows:
>
> [root at hp-dl120gen9-01 ~]# TZ=:CST6 date
> Thu Jul  9 06:50:29 CST 2015

TZ=':CST6' uses an implementation-specific extension to POSIX.  Often in this 
case implementations ignore the ':' so it's equivalent to TZ='CST6'.  tzcode 
does that, and I assume glibc does too.

With TZ='CST6', the time zone is six hours behind UTC (i.e., west of Greenwich), 
like this:

$ date -u; TZ=CST6 date
Wed Jul 29 22:52:44 UTC 2015
Wed Jul 29 16:52:44 CST 2015

> But this is not consitent if tzdata package is update.

A TZ setting like 'CST6' is a string specified by POSIX, and should work 
correctly regardless of what's in tzdata.

> Also this does take into account DST change?

TZ='CST6' has no DST.

For more about POSIX TZ settings, please see:

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03