Test optional timezone filtered by country with [form.timezone.filterout].Because of [form.maybenull=true] he initial timezone would be empty but [form.timezone.guess=true] makes it guessed from the user locale
Test timezone filtered by country with [form.timezone.filterout]. If the country changes and the selected timezone is not in the country group the timezone is unselected In this case the initial timezone is guessed from the user locale, use form.mayben...