def build_timezone_identifiers
      data = data_timezone_identifiers
      linked = linked_timezone_identifiers
      linked.empty? ? data : (data + linked).sort!.freeze
    end
def build_timezone_identifiers
      data = data_timezone_identifiers
      linked = linked_timezone_identifiers
      linked.empty? ? data : (data + linked).sort!.freeze
    end
Combines {data_timezone_identifiers} and {linked_timezone_identifiers}
 to create an `Array` containing all valid time zone identifiers. If
 {linked_timezone_identifiers} is empty, the {data_timezone_identifiers}
 instance is returned.

 The returned `Array` is frozen. The identifiers are sorted according to
 `String#<=>`.

 @return [Array<String>] an `Array` containing all valid time zone
   identifiers.