validates_inclusion_of :time_zone, in: ActiveSupport::TimeZone.zones_map(&:name)

validates :time_zone,
  inclusion: {
    in: ActiveSupport::TimeZone.zones_map.keys
  }

validates_inclusion_of :time_zone,
   :in => ActiveSupport::TimeZone.zones_map(&:name).keys,
   :message => "is not a valid time zone"