The clock documentation says the default format is {%a %b %d %H:%M:%S %z %Y}.  But it looks like the default format uses %Z, not %z.

% clock format [clock seconds]
Tue Sep 01 22:29:30 PDT 2015
% clock format [clock seconds] -format {%a %b %d %H:%M:%S %z %Y}
Tue Sep 01 22:30:47 -0700 2015