I'm not sure there's a portable way to do what you want, apart from Time::Date or similar, or compiling a database of timezones and offsets and using that.

That said, on my system zdump(8) looks helpful:

sgifford@sglaptop sgifford$ /usr/sbin/zdump US/Michigan Zulu
US/Michigan  Fri Oct 24 03:16:56 2003 EDT
Zulu         Fri Oct 24 07:16:56 2003 UTC