[environment variable listing the timezones and gmt offset for the current context]

MAKR attempts (2009-01-30): POSIX has a definition of the environment variable TZ (taken from [L1 ]) that goes like this: This variable shall represent timezone information. The contents of the environment variable named TZ shall be used by [...] functions, and by various utilities, to override the default timezone. The value of TZ has one of the two forms (spaces inserted for clarity):



    std offset dst offset, rule

If TZ is of the first format (that is, if the first character is a colon), the characters following the colon are handled in an implementation-defined manner.

The timezone wiki page has what that means for Tcl. For an explanation of the second form see the above referenced page from the POSIX standard.