It depends on the system. I'm gonna guess you are talking about windows only, but in case you aren't..
Mon Jan 5 02:35:37 2004[calligraphy:~] sporty% setenv TZ CET
[calligraphy:~] sporty% perl -e 'print scalar localtime'
Mon Jan 5 03:35:46 2004[calligraphy:~] sporty% setenv TZ MST
[calligraphy:~] sporty% perl -e 'print scalar localtime'
Sun Jan 4 19:35:51 2004[calligraphy:~] sporty%
It's because of what's in my /usr/share/zoneinfo/ dir .. so if I created a new one, I could use that, without the number.
Play that funky music white boy..