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%
[download]

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.