That code won't compile as 'p' and 'l' are not declared variables - I guess they should be 'panel' and 'label'. Once changed to those names it works for me, every new timezone I entered changed the displayed time although I'm not convinced that manually entering a timezone via combobox is the best way to do this.
Variables should be declared as locally as possible so for example the variable 's' which is only used in the actionPerformed() method should be declared in that method rather than as an instance variable.
Instance variables should be declared as private unless there is a very good reason that they need wider visibility.
Class names should start with an upper case letter as per the Java Naming Convention.
The knights of nee want a shrubbery. And a tiny ad:
Devious Experiments for a Truly Passive Greenhouse!