Now that we can detect if there was a timezone, implement these functions.

Returns the timezone component, if any, of $arg as an
xs:string. The $arg is of type xs:dateTime, xs:date and
xs:time respectively.
If $arg has a timezone component, then the result is a
string in the format of an ISO Time zone designator.
Interpreted as an offset from UTC, its value may range from
+14:00 to -14:00 hours, both inclusive. The UTC time zone
is represented as "+00:00". If the $arg has no timezone
component, then "" (empty string) is returned.