Easiest way? Given you're stating 500 errors, this is a web app. Provide a drop-down list of valid timezones instead of a text field and all of your problems go away.

If that's not feasible, populate a hash where the keys are valid zones, check for exists and spit an error if necessary (or build logic to guess... which I'd recommend against, unless, of course, a guess or a default is acceptable).