date(1) treats TZ '' differently from unset

[Ed Avis]

On a system where local time is +0100:

% (unset TZ; date +%z)
+0100
% (TZ= date +%z)
+0000

Are you sure this is correct?  I would have expected the empty string
for TZ to act the same as having that variable unset, or otherwise to
produce a warning that TZ was set to a bad value.

% date --version
date (coreutils) 5.0
Written by David MacKenzie.

Copyright (C) 2003 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

-- 
Ed Avis <address@hidden>